View source for Neutrino

{{short description|Elementary particle with extremely low mass}}
{{distinguish|neutron|neuron|neutralino}}
{{Other uses}}
{{use dmy dates|date=March 2023}}
{{Infobox particle
 | name = Neutrino
 | image = FirstNeutrinoEventAnnotated.jpg
 | image_size = 280px
 | caption = The first use of a hydrogen [[bubble chamber]] to detect neutrinos, on 13&nbsp;November 1970, at [[Argonne National Laboratory]]. Here a neutrino hits a proton in a hydrogen atom; the collision occurs at the point where three tracks emanate on the right of the photograph.
 | num_types = 3&nbsp;types: {{nowrap|electron neutrino ({{math|{{SubatomicParticle|Electron neutrino}}}})}}, {{nowrap|muon neutrino ({{math|{{SubatomicParticle|Muon neutrino}}}})}}, and {{nowrap|tau neutrino ({{math|{{SubatomicParticle|Tau neutrino}}}})}}
 | composition = [[Elementary particle]]
 | group =   [[Lepton]]s, antileptons
 | statistics = [[Fermionic]]
 | generation = First ({{math|{{SubatomicParticle|Electron neutrino}}}}), second ({{math|{{SubatomicParticle|Muon neutrino}}}}), and third ({{math|{{SubatomicParticle|Tau neutrino}}}})
 | interaction = [[Weak interaction]] and [[gravitation]]
 | particle = {{nowrap|spin: {{small|{{sfrac|±| 1 |2}}}}{{math|''ħ''}},}} {{nowrap|[[Chirality (physics)|chirality]]: '''L'''eft,}} {{nowrap|[[weak isospin]]: +{{small|{{sfrac| 1 |2}}}},}} {{nowrap|[[lepton number|lepton nr.]]: +1}}, {{nowrap|[[Flavour (particle physics)|"flavor"]] in {{mset| {{math|e, &mu;, &tau;}} }} }}
 | antiparticle = {{nowrap|spin: {{small|{{sfrac|±| 1 |2}}}}{{math|''ħ''}},}} {{nowrap|[[Chirality (physics)|chirality]]: '''R'''ight,}} {{nowrap|[[weak isospin]]: −{{small|{{sfrac| 1 |2}}}},}} {{nowrap|[[lepton number|lepton nr.]]: −1}}, {{nowrap|[[Flavour (particle physics)|"flavor"]] in {{mset| {{overline|{{math|e}}}}, {{overline|{{math|&mu;}}}}, {{overline|{{math|&tau;}}}} }} }}
 | theorized = {{plainlist|
* {{math|{{SubatomicParticle|Electron neutrino}}}}, [[electron neutrino]]: [[Wolfgang Pauli]] (1930)
* {{math|{{SubatomicParticle|Muon neutrino}}}}, [[muon neutrino]]: late 1940s
* {{math|{{SubatomicParticle|Tau neutrino}}}}, [[tau neutrino]]: mid-1970s
}}
 | discovered = {{plainlist|
* {{math|{{SubatomicParticle|Electron neutrino}}}}: [[Clyde Cowan]], [[Frederick Reines]] (1956)
* {{math|{{SubatomicParticle|Muon neutrino}}}}: [[Leon Lederman]], [[Melvin Schwartz]] and [[Jack Steinberger]] (1962)
* {{math|{{SubatomicParticle|Tau neutrino}}}}: [[DONUT|DONUT collaboration]] (2000)
}}
 | symbol = {{math|{{SubatomicParticle|Electron neutrino}}}}&nbsp;, {{math|{{SubatomicParticle|Muon neutrino}}}}&nbsp;, {{math|{{SubatomicParticle|Tau neutrino}}}}&nbsp;, {{math|{{SubatomicParticle|Electron antineutrino}}}}&nbsp;, {{math|{{SubatomicParticle|Muon antineutrino}}}}&nbsp;, {{math|{{SubatomicParticle|Tau antineutrino}}}}
 | mass = <&nbsp;{{val|0.120|u=eV/c2}} (<&nbsp;{{val|2.14|e=−37|u=kg}}), cosmological limit, sum of 3&nbsp;[[Flavour (particle physics)|"flavors"]]<ref name=PDG-2024>
{{cite journal
 |first1=K.A. |last1=Olive
 |year=2024
 |title=Sum of neutrino masses
 |editor1-first=S. |editor1-last=Navas
 |display-editors=etal
 |collaboration=[[Particle Data Group]]
 |journal=[[Physical Review D]]
 |volume=110 |page=030001
 |url=https://pdg.lbl.gov/2024/reviews/rpp2024-rev-sum-neutrino-masses.pdf
 |via=[[Lawrence Berkeley Laboratory]] (lbl.gov)
}}
</ref>
 | decay_time      =
 | decay_particle  =
 | electric_charge = 0&nbsp;[[elementary charge|''e'']]
 | color_charge    = 
 | spin            = {{sfrac| 1 |2}}{{math|''ħ''}}
 | X_charge        = −3
 | weak_isospin    = {{nowrap|LH: +{{sfrac| 1 |2}}}}, {{nowrap|RH: 0}}
 | weak_hypercharge= {{nowrap|LH: −1}}, {{nowrap|RH: 0}}
 | B-L             = −1
}}
A '''neutrino''' ({{IPAc-en|nj|uː|ˈ|t|r|iː|n|oʊ}} {{Respell|new|TREE|noh}}; denoted by the Greek letter [[Nu (letter)|{{math|ν}}]]) is an [[elementary particle]] that interacts  via the [[weak interaction]] and [[gravity]].<ref name=Close-2010-νν/><ref name=Jayawrdh-2015-νhunt/> The neutrino is so named because it is [[electric charge|electrically]] neutral and because its [[rest mass]] is so small (''[[List of diminutives by language#Italian|-ino]]'') that it was long thought to be [[Massless particle|zero]]. The rest [[mass]] of the neutrino is much smaller than that of the other known elementary particles (excluding [[massless particles]]).<ref name=Mertens-2016-mν>
{{cite journal
 |first1=Susanne |last1=Mertens
 |year=2016
 |title=Direct neutrino mass experiments
 |journal=[[Journal of Physics: Conference Series]]
 |volume=718 |issue=2 |article-number=022013
 |arxiv=1605.01579 |bibcode=2016JPhCS.718b2013M
 |doi=10.1088/1742-6596/718/2/022013 |s2cid=56355240
}}
</ref>
The weak force has a very short range, the gravitational interaction is extremely weak due to the very small mass of the neutrino, and neutrinos do not participate in the [[Electromagnetism|electromagnetic interaction]] or the [[strong interaction]].<ref name=Overbye-2020-04-15-BBν>
{{cite news
 |last=Overbye |first=Dennis |author-link=Dennis Overbye
 |date=15 April 2020
 |title=Why the Big Bang produced something rather than nothing
 |quote=How did matter gain the edge over antimatter in the early universe? Maybe, just maybe, neutrinos.
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/2020/04/15/science/physics-neutrino-antimatter-ichikawa-t2k.html
 |url-status=live  |access-date=16 April 2020
 |archive-url=https://web.archive.org/web/20200514083406/https://www.nytimes.com/2020/04/15/science/physics-neutrino-antimatter-ichikawa-t2k.html
 |archive-date=14 May 2020
}}
</ref>
Consequently, neutrinos typically pass through normal matter unimpeded and with no detectable effect.<ref name=Close-2010-νν>
{{cite book
 |last=Close |first=Frank |author-link=Frank Close
 |year=2010
 |title=Neutrinos
 |edition=softcover
 |publisher=[[Oxford University Press]]
 |isbn=978-0-199-69599-7
}}
</ref><ref name=Jayawrdh-2015-νhunt>
{{cite book
 |last=Jayawardhana |first=Ray |author-link=Ray Jayawardhana
 |year=2015
 |title=The Neutrino Hunters: The chase for the ghost particle and the secrets of the universe
 |publisher=[[Oneworld Publications]]
 |edition=softcover |isbn=978-1-780-74647-0
}}
</ref>

[[Weak interactions]] create neutrinos in one of three leptonic [[Flavor (particle physics)|flavors]]:
# [[electron neutrino]], {{math|{{SubatomicParticle|Electron neutrino}}}}
# [[muon neutrino]], {{math|{{SubatomicParticle|Muon neutrino}}}}
# [[tau neutrino]], {{math|{{SubatomicParticle|Tau neutrino}}}}

Each flavor is associated with the correspondingly named charged [[lepton]].<ref>{{cite journal |last1=Nakamura |first1=Kengo |last2=Petcov |first2=Serguey Todorov |year=2016 |title=Neutrino mass, mixing, and oscillations |journal=[[Chinese Physics C]] |volume=40 |page=100001 |url=http://pdg.lbl.gov/2016/reviews/rpp2016-rev-neutrino-mixing.pdf |access-date=13 December 2016 |archive-date=17 April 2018 |archive-url=https://web.archive.org/web/20180417202541/http://pdg.lbl.gov/2016/reviews/rpp2016-rev-neutrino-mixing.pdf |url-status=live }}</ref> Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses with different values (all tiny, the smallest of which could be zero<ref>{{cite journal |last1 = Boyle |first1 = Latham |last2 = Finn |first2 = Kiernan |last3 = Turok |first3 = Neil |year = 2022 |title = The Big Bang, CPT, and neutrino dark matter |journal = Annals of Physics |volume = 438 |article-number = 168767 |doi = 10.1016/j.aop.2022.168767 |arxiv = 1803.08930 |bibcode = 2022AnPhy.43868767B |s2cid = 119252778}}</ref>), but the three masses do not uniquely correspond to the three flavors: A neutrino created with a specific flavor is a specific mixture of all three mass states (a ''[[quantum superposition]]''). Similar to some [[neutral particle oscillation|other neutral particles]], [[Neutrino oscillation|neutrinos oscillate]] between different flavors in flight as a consequence. For example, an electron neutrino produced in a [[beta decay]] reaction may interact in a distant detector as a muon or tau neutrino.<ref name=Grossman-Lipkin-1997>{{cite journal |last1=Grossman |first1=Yuval |last2=Lipkin |first2=Harry J. |author2-link=Harry J. Lipkin |year=1997 |title=Flavor oscillations from a spatially localized source — A simple general treatment |journal=[[Physical Review|Physical Review D]] |volume=55 |issue=5 |page=2760 |arxiv=hep-ph/9607201 |bibcode=1997PhRvD..55.2760G |doi=10.1103/PhysRevD.55.2760 |s2cid=9032778}}</ref><ref name=Bilenky-2016-ν-osc>{{cite journal |first=Samoil M. |last=Bilenky |year=2016 |arxiv=1602.00170 |bibcode=2016NuPhB.908....2B |doi=10.1016/j.nuclphysb.2016.01.025 |title=Neutrino oscillations: From a historical perspective to the present status |journal=[[Nuclear Physics (journal)|Nuclear Physics B]] |volume=908 |pages=2–13 |s2cid=119220135}}</ref> The three mass values are not yet known as of 2024, but laboratory experiments and [[Cosmology|cosmological]] observations have determined the differences of their squares,<ref name=Capozzi-Lisi-Marrone-etal-2016>{{cite journal |last1=Capozzi |first1=Francesco |last2=Lisi |first2=Eligio |last3=Marrone |first3=Antonio |last4=Montanino |first4=Daniele |last5=Palazzo |first5=Antonio |year=2016 |title=Neutrino masses and mixings: Status of known and unknown 3ν parameters |journal=Nuclear Physics B |volume=908 |pages=218–234 |arxiv=1601.07777 |bibcode=2016NuPhB.908..218C |doi=10.1016/j.nuclphysb.2016.02.016 |s2cid=119292028}}</ref> an upper limit on their sum (<&nbsp;{{val|0.120|u=eV/c2}}),<ref name=PDG-2024/> and an upper limit on the mass of the electron neutrino.<ref name=KATRIN-2022-NatPhys/> Neutrinos are [[fermion]]s, which have [[spin-1/2|spin of {{sfrac| 1 |2}}{{math|''ħ''}}]].

For each neutrino, there also exists a corresponding [[antiparticle]], called an [[#Antineutrinos|''antineutrino'']], which also has spin of {{sfrac| 1 |2}}{{math|''ħ''}} and no electric charge. Antineutrinos are distinguished from neutrinos by having opposite-signed [[lepton number]] and [[weak isospin]], and right-handed instead of left-handed chirality. To conserve total lepton number (in nuclear beta decay), electron neutrinos only appear together with [[positron]]s (anti-electrons) or electron-antineutrinos, whereas electron antineutrinos only appear with electrons or electron neutrinos.<ref name=FourPeaksAZ-ghostν>{{cite web |title=Ghostlike neutrinos |website=particlecentral.com |publisher=Four Peaks Technologies |location=Scottsdale, AZ |url=http://www.particlecentral.com/neutrinos_page.html |access-date=24 April 2016 |archive-date=24 March 2016 |archive-url=https://web.archive.org/web/20160324131645/http://www.particlecentral.com/neutrinos_page.html }}</ref><ref name=HypPhys-GSU-consℓ>{{cite web |title=Conservation of lepton number |series=HyperPhysics / particles |publisher=Georgia State University |url=http://hyperphysics.phy-astr.gsu.edu/hbase/particles/parint.html#c3 |access-date=24 April 2016 |archive-date=27 April 2016 |archive-url=https://web.archive.org/web/20160427004314/http://hyperphysics.phy-astr.gsu.edu/hbase/particles/parint.html#c3 |url-status=live }}</ref>

Neutrinos are created by various [[radioactive decay]]s; the following list is not exhaustive, but includes some of those processes:
* [[beta decay]] of [[atomic nuclei]] or [[hadron]]s
* natural [[nuclear reaction]]s such as those that take place in the core of a [[star]]
* artificial nuclear reactions in [[nuclear reactor]]s, [[nuclear bomb]]s, or [[particle accelerator]]s
* during a [[supernova]]
* during the spin-down of a [[neutron star]]
* when [[cosmic ray]]s or accelerated particle beams strike atoms

The majority of neutrinos which are detected about the Earth are from nuclear reactions inside the Sun. At the surface of the Earth, the flux is about 65&nbsp;billion ({{val|6.5|e=10}}) [[solar neutrino]]s, per second per square centimeter.<ref name=Armitage-JILA-2003-sol-ν>
{{cite web
 |first=Philip
 |last=Armitage
 |year=2003
 |title=Solar neutrinos
 |department=JILA
 |publisher=University of Colorado
 |location=Boulder, CO
 |url=http://jila.colorado.edu/~pja/astr3730/lecture21.pdf
 |access-date=24 April 2016
 |archive-date=28 March 2016
 |archive-url=https://web.archive.org/web/20160328131827/http://jila.colorado.edu/~pja/astr3730/lecture21.pdf
 |url-status=live
}}</ref><ref name=Bahcall-Serenelli-Basu-2005>
{{cite journal
 |last1=Bahcall |first1=John N. |author-link1=John N. Bahcall
 |last2=Serenelli |first2=Aldo M.
 |last3=Basu |first3=Sarbani |author-link3=Sarbani Basu
 |year=2005
 |title=New solar opacities, abundances, helioseismology, and neutrino fluxes
 |journal=[[The Astrophysical Journal]]
 |volume=621 |issue=1 |pages=L85–L88
 |s2cid=1374022 |arxiv=astro-ph/0412440
 |bibcode=2005ApJ...621L..85B |doi=10.1086/428929
}}
</ref> Neutrinos can be used for [[tomography]] of the interior of the Earth.<ref name=Millhouse-Lipkin-2013-tomog>{{cite journal
 |first1=Margaret A.
 |last1=Millhouse
 |first2=David C.
 |last2=Lipkin
 |year=2013
 |title=Neutrino tomography
 |journal=[[American Journal of Physics]]
 |volume=81
 |issue=9
 |pages=646–654
 |bibcode=2013AmJPh..81..646M
 |doi=10.1119/1.4817314
 |url=http://soundideas.pugetsound.edu/cgi/viewcontent.cgi?article=2059&context=faculty_pubs
 |archive-date=20 December 2016
 |access-date=14 December 2016
 |archive-url=https://web.archive.org/web/20161220161826/http://soundideas.pugetsound.edu/cgi/viewcontent.cgi?article=2059&context=faculty_pubs
 }}</ref><ref name="PINGU-2014-LoIntt">
{{cite report |title=The Precision IceCube Next Generation Upgrade (PINGU) |last=Aartsen |first=M. G. |arxiv=1401.2046 |collaboration=The IceCube-PINGU Collaboration |year=2014 |series=Letter of Intent}}
</ref>

== History ==

=== Pauli's proposal ===
The neutrino{{efn|
More specifically, Pauli postulated what is now called the ''electron neutrino''. Two other types were discovered later: see ''[[#Neutrino_flavors_anchor|Neutrino flavor]]'' below.
}} was postulated first by [[Wolfgang Pauli]] in 1930 to explain how beta decay could conserve [[conservation of energy|energy]], [[conservation of momentum|momentum]], and [[conservation of angular momentum|angular momentum]] ([[Spin (physics)|spin]]). In contrast to [[Niels Bohr]], who proposed a statistical version of the conservation laws to explain the observed [[Beta decay#Neutrinos|continuous energy spectra in beta decay]], Pauli hypothesized an undetected particle that he called a "neutron", using the same ''-on'' ending employed for naming both the [[proton]] and the [[electron]]. He considered that the new particle was emitted from the nucleus together with the electron or beta particle in the process of beta decay and had a mass similar to the electron.<ref name=Brown-1978-idea-ν>
{{cite journal
 |last=Brown 
 |first=Laurie M. 
 |author-link=Laurie Brown (physicist)
 |year=1978
 |title=The idea of the neutrino
 |journal=[[Physics Today]]
 |volume=31 
 |issue=9 
 |pages=23–28
 |bibcode=1978PhT....31i..23B 
 |doi=10.1063/1.2995181
}}
</ref>{{efn|
[[Niels Bohr]] was notably opposed to this interpretation of beta decay—he was ready to accept that energy, momentum, and angular momentum were not conserved quantities at the atomic level.
}}

[[James Chadwick]] discovered a much more massive neutral nuclear particle in 1932 and named it a [[neutron]] also, leaving two kinds of particles with the same name. The word "neutrino" entered the scientific vocabulary through [[Enrico Fermi]], who used it during a conference in Paris in July&nbsp;1932 and at the Solvay Conference in October&nbsp;1933, where Pauli also employed it. The name (the [[Italian language|Italian]] equivalent of "little neutral one") was jokingly coined by [[Edoardo Amaldi]] during a conversation with Fermi at the Institute of Physics of via Panisperna in Rome, in order to distinguish this light neutral particle from Chadwick's heavy neutron.<ref>
{{cite journal
 |last=Amaldi 
 |first=Edoardo
 |author-link=Edoardo Amaldi
 |year=1984
 |title=From the discovery of the neutron to the discovery of nuclear fission
 |journal=[[Physics Reports]]
 |volume=111 |issue=1–4 |page=306
 |bibcode=1984PhR...111....1A
 |doi=10.1016/0370-1573(84)90214-X
}}
</ref>

In [[Fermi's interaction|Fermi's theory of beta decay]], Chadwick's large neutral particle could decay to a proton, electron, and the smaller neutral particle (now called an ''electron antineutrino''):
: {{math| {{SubatomicParticle|Neutron0}} → {{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron-}} + {{SubatomicParticle|Electron antineutrino}} }}

Fermi's paper, written in 1934,<ref name=Fermi-1934/> unified Pauli's neutrino with [[Paul Dirac]]'s [[positron]] and [[Werner Heisenberg]]'s neutron–proton model and gave a solid theoretical basis for future experimental work.<ref name=Fermi-1934>
{{cite journal
 |last=Fermi
 |first=Enrico
 |author-link=Enrico Fermi
 |date=March 1934
 |title=Versuch einer Theorie der β-Strahlen. I
 |trans-title=Search for a theory of β-decay. I
 |journal=[[Zeitschrift für Physik A]]
 |language=de
 |volume=88
 |issue=3–4
 |pages=161–177
 |bibcode=1934ZPhy...88..161F
 |doi=10.1007/BF01351864
 |s2cid=125763380
}}</ref><ref>
{{cite journal
 |last=Wilson
 |first=Fred L.
 |date=1 December 1968
 |title=Fermi's theory of beta decay
 |journal=[[American Journal of Physics]]
 |volume=36
 |issue=12
 |pages=1150–1160
 |bibcode=1968AmJPh..36.1150W
 |doi=10.1119/1.1974382
 |url=http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=FermiBetaDecay1934.pdf;version=1
 |access-date=5 October 2011
 |archive-date=12 May 2013
 |archive-url=https://web.archive.org/web/20130512011303/http://microboone-docdb.fnal.gov/cgi-bin/RetrieveFile?docid=953;filename=FermiBetaDecay1934.pdf;version=1
 |url-status=live
}}</ref><ref name=Close-2012-ν>
{{cite book
 |last=Close
 |first=Frank
 |date=12 April 2012
 |orig-date=2010
 |title=Neutrino
 |publisher=[[Oxford University Press]]
 |page=24
 |isbn=978-0-19-969599-7
}}</ref><!--{{rp|page=24}}-->

By 1934, there was experimental evidence against Bohr's idea that energy conservation is invalid for beta decay: At the [[Solvay conference]] of that year, measurements of the energy spectra of beta particles (electrons) were reported, showing that there is a strict limit on the energy of electrons from each type of beta decay. Such a limit is not expected if the conservation of energy is invalid, in which case any amount of energy would be statistically available in at least a few decays. The natural explanation of the beta decay spectrum as first measured in 1934 was that only a limited (and conserved) amount of energy was available, and a new particle was sometimes taking a varying fraction of this limited energy, leaving the rest for the beta particle. Pauli made use of the occasion to publicly emphasize that the still-undetected "neutrino" must be an actual particle.<ref name=Close-2012-ν/>{{rp|page=25}} The first evidence of the reality of neutrinos came in 1938 via simultaneous cloud-chamber measurements of the electron and the recoil of the nucleus.<ref>
{{cite news
 |title=Cloud-chamber test finds neutrino 'real'
 |date=22 May 1938
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/1938/05/22/archives/cloudchamber-test-finds-neutrino-real-drs-crane-and-halpern-decide.html?sq=%2522H.%2520Richard%2520Crane%2522&scp=1&st=cse
 |quote=Drs.&nbsp;Crane and Halpern decide it is no mere hypothesis
}}
</ref>

=== Direct detection ===
[[File:Clyde Cowan.jpg|thumb|upright=1.1|Fred Reines and Clyde Cowan conducting the neutrino experiment c. 1956]]
In 1942, [[Wang Ganchang]] first proposed the use of [[Electron capture|beta capture]] to experimentally detect neutrinos.<ref>
{{cite journal
 |last=Wang |first=Kan Chang |author-link=Wang Ganchang
 |year=1942
 |title=A suggestion on the detection of the neutrino
 |journal=[[Physical Review]]
 |volume=61 |issue=1–2 |page=97
 |bibcode=1942PhRv...61...97W |doi=10.1103/PhysRev.61.97
}}
</ref> In the 20&nbsp;July 1956 issue of [[Science (journal)|''Science'']], [[Clyde Cowan]], [[Frederick Reines]], Francis B. "Kiko" Harrison, Herald W. Kruse, and Austin D. McGuire published confirmation that they had detected the neutrino,<ref>
{{cite journal
 |last1=Cowan |first1=Clyde L. Jr. |author1-link=Clyde Cowan
 |last2=Reines |first2=Frederick |author2-link=Frederick Reines
 |last3=Harrison |first3=Francis B. "Kiko"
 |last4=Kruse |first4=Herald W.
 |last5=McGuire |first5=Austin D.
 |year=1956
 |title=Detection of the free neutrino: A confirmation
 |journal=[[Science (journal)|Science]]
 |volume=124 |issue=3212 |pages=103–104
 |bibcode=1956Sci...124..103C
 |doi=10.1126/science.124.3212.103 |pmid=17796274
}}
</ref><ref>
This source reproduces the 1956 paper:<br />{{cite book |last=Winter |first=Klaus |url=https://books.google.com/books?id=v_tiL2NlfvMC&pg=PA38 |title=Neutrino Physics |publisher=[[Cambridge University Press]] |year=2000 |isbn=978-0-521-65003-8 |pages=38&nbsp;ff}}
</ref> a result that was rewarded almost forty years later with the [[Nobel Prize in Physics|1995 Nobel Prize]].<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=1995
 |publisher=[[The Nobel Foundation]]
 |url=https://nobelprize.org/nobel_prizes/physics/laureates/1995/
 |access-date=29 June 2010
 |archive-date=30 June 2018
 |archive-url=https://web.archive.org/web/20180630025246/https://www.nobelprize.org/nobel_prizes/physics/laureates/1995/
 |url-status=live
}}</ref>

In this experiment, now known as the [[Cowan–Reines neutrino experiment]], antineutrinos created in a nuclear reactor by beta decay reacted with protons to produce [[neutron]]s and positrons:
: {{math| {{SubatomicParticle|Electron antineutrino}} + {{SubatomicParticle|Proton+}} → {{SubatomicParticle|Neutron0}} + {{SubatomicParticle|Electron+}} }}

The positron quickly finds an electron, and they [[Annihilation|annihilate]] each other. The two resulting [[gamma ray]]s (γ) are detectable. The neutron can be detected by its capture on an appropriate nucleus, releasing a gamma ray. The coincidence of both events—positron annihilation and neutron capture—gives a unique signature of an antineutrino interaction.

In February&nbsp;1965, the first neutrino found in nature was identified by a group including Frederick Reines and [[Friedel Sellschop]].<ref>{{cite web |url=https://www.space.com/what-are-neutrinos |title=What are neutrinos? |date=2022-09-21 |access-date=2023-12-22 |website=Space.com |last=Cooper |first=Keith |archive-date=22 December 2023 |archive-url=https://web.archive.org/web/20231222154103/https://www.space.com/what-are-neutrinos |url-status=live }}</ref><ref>{{cite journal |last1=Reines |first1=F. |last2=Crouch |first2=M. F. |last3=Jenkins |first3=T. L. |last4=Kropp |first4=W. R. |last5=Gurr |first5=H. S. |last6=Smith |first6=G. R. |last7=Sellschop |first7=J. P. F. |last8=Meyer |first8=B. |title=Evidence for High-Energy Cosmic-Ray Neutrino Interactions |journal=Physical Review Letters |date=30 August 1965 |volume=15 |issue=9 |pages=429–433 |doi=10.1103/PhysRevLett.15.429 |bibcode=1965ICRC....2.1051R |url=https://adsabs.harvard.edu/full/1965ICRC....2.1051R |access-date=22 December 2023}}</ref> The experiment was performed in a specially prepared chamber at a depth of 3&nbsp;km in the [[East Rand Mine|East Rand ("ERPM") gold mine]] near [[Boksburg]], South Africa. A plaque in the main building commemorates the discovery. The experiments also implemented a primitive neutrino astronomy and looked at issues of neutrino physics and weak interactions.<ref>
{{cite journal
 |last1=Johnson |first1=C.D.
 |last2=Tegen |first2=Rudolph
 |title=The ''little neutral one'': An overview of the neutrino
 |date=January 1999
 |journal=[[South African Journal of Science]]
 |volume=95
 |number=95 |pages=13–20
 |hdl=10520/AJA00382353_7822
 |url=https://journals.co.za/doi/pdf/10.10520/AJA00382353_7822
}}
</ref>

=== Neutrino flavor <span class="anchor" id="Neutrino_flavors_anchor"></span> ===
The antineutrino discovered by [[Clyde Cowan]] and [[Frederick Reines]] was the antiparticle of the electron neutrino.

In 1962, [[Leon M. Lederman]], [[Melvin Schwartz]], and [[Jack Steinberger]] showed that more than one type of neutrino exists by first detecting interactions of the [[muon]] neutrino (already hypothesised with the name ''neutretto''),<ref>
{{cite journal
 |first=Ivan V. |last=Aničin |author-link=Ivan Aničin
 |year=2005
 |title=The neutrino – its past, present, and future
 |journal=SFIN (Institute of Physics, Belgrade) Year&nbsp;XV
 |series=A: Conferences
 |id=No. A (00)
 |volume=2 |issue=2002 |pages=3–59
 |arxiv=physics/0503172 |bibcode=2005physics...3172A
}}
</ref> which earned them the [[Nobel Prize in Physics|1988 Nobel Prize in Physics]].

When the third type of lepton, the [[tau (particle)|tau]], was discovered in 1975 at the [[Stanford Linear Accelerator Center]], it was also expected to have an associated neutrino (the tau neutrino). The first evidence for this third neutrino type came from the observation of missing energy and momentum in tau decays analogous to the beta decay leading to the discovery of the electron neutrino. The first detection of tau neutrino interactions was announced in 2000 by the [[DONUT|DONUT collaboration]] at [[Fermilab]]; its existence had already been inferred by both theoretical consistency and experimental data from the [[Large Electron–Positron Collider]].<ref>
{{cite news
 |title=Physicists find first direct evidence for Tau neutrino at Fermilab
 |publisher=[[Fermilab]]
 |date=20 July 2000
 |quote=In 1989, experimenters at CERN found proof that the tau neutrino is the third and last light neutrino of the Standard Model, but a direct observation was not yet feasible.
 |url=http://www.fnal.gov/pub/presspass/press_releases/donut.html
 |access-date=9 October 2015
 |archive-date=20 October 2016
 |archive-url=https://web.archive.org/web/20161020025730/http://www.fnal.gov/pub/presspass/press_releases/donut.html
 |url-status=live
}}</ref>

=== Solar neutrino problem ===
{{Main|Solar neutrino problem}}
In the 1960s, the now-famous [[Homestake experiment]] made the first measurement of the flux of electron neutrinos arriving from the core of the Sun and found a value that was between one third and one half the number predicted by the [[Standard Solar Model]]. This discrepancy, which became known as the [[solar neutrino problem]], remained unresolved for some thirty years, while possible problems with both the experiment and the solar model were investigated, but none could be found. Eventually, it was realized that both were actually correct and that the discrepancy between them was due to neutrinos being more complex than was previously assumed. It was postulated that the three neutrinos had nonzero and slightly different masses, and could therefore oscillate into undetectable flavors on their flight to the Earth. This hypothesis was investigated by a new series of experiments, thereby opening a new major field of research that still continues. Eventual confirmation of the phenomenon of neutrino oscillation led to two Nobel prizes, one to [[Raymond Davis, Jr.|R. Davis]], who conceived and led the Homestake experiment and [[Masatoshi Koshiba]] of Kamiokande, whose work confirmed it, and one to [[Takaaki Kajita]] of [[Super-Kamiokande]] and [[Arthur B. McDonald|A.B. McDonald]] of [[Sudbury Neutrino Observatory]] for their joint experiment, which confirmed the existence of all three neutrino flavors and found no deficit.<ref name=CERN-2001-12-04-SNO/>

=== Oscillation ===
{{main|Neutrino oscillation}}
A practical method for investigating neutrino oscillations was first suggested by [[Bruno Pontecorvo]] in 1957 using an analogy with [[kaon]] oscillations; over the subsequent 10&nbsp;years, he developed the mathematical formalism and the modern formulation of vacuum oscillations. In 1985 [[Stanislav Mikheyev]] and [[Alexei Smirnov (physicist)|Alexei Smirnov]] (expanding on 1978 work by [[Lincoln Wolfenstein]]) noted that flavor oscillations can be modified when neutrinos propagate through matter. This so-called [[Mikheyev–Smirnov–Wolfenstein effect]] (MSW effect) is important to understand because many neutrinos emitted by fusion in the Sun pass through the dense matter in the [[solar core]] (where essentially all solar fusion takes place) on their way to detectors on Earth.

Starting in 1998, experiments began to show that solar and atmospheric neutrinos change flavors (see ''[[Super-Kamiokande]]'' and ''[[Sudbury Neutrino Observatory]]''). This resolved the solar neutrino problem: the electron neutrinos produced in the Sun had partly changed into other flavors which the experiments could not detect.

Although individual experiments, such as the set of solar neutrino experiments, are consistent with non-oscillatory mechanisms of neutrino flavor conversion, taken altogether, neutrino experiments imply the existence of neutrino oscillations. Especially relevant in this context are the reactor experiment [[KamLAND]] and the accelerator experiments such as [[MINOS]]. The KamLAND experiment has indeed identified oscillations as the neutrino flavor conversion mechanism involved in the solar electron neutrinos. Similarly MINOS confirms the oscillation of atmospheric neutrinos and gives a better determination of the mass squared splitting.<ref>
{{cite journal
 |last1=Maltoni |first1=Michele
 |last2=Schwetz |first2=Thomas
 |last3=Tórtola |first3=Mariam A.
 |last4=Valle |first4=José W.F. |author-link4=José W. F. Valle
 |year=2004
 |title=Status of global fits to neutrino oscillations
 |journal=[[New Journal of Physics]]
 |volume=6 |issue=1 |page=122
 |arxiv=hep-ph/0405172 |bibcode=2004NJPh....6..122M
 |doi=10.1088/1367-2630/6/1/122 |s2cid=119459743
}}
</ref> Takaaki Kajita of Japan, and Arthur B. McDonald of Canada, received the 2015 Nobel Prize for Physics for their landmark finding, theoretical and experimental, that neutrinos can change flavors.

=== Cosmic neutrinos ===
{{main|cosmic neutrino background|diffuse supernova neutrino background}}
As well as specific sources, a general background level of neutrinos is expected to pervade the universe, theorized to occur due to two main sources.

; Cosmic neutrino background (Big Bang-originated) :

Around 1&nbsp;second after the [[Big Bang]], neutrinos decoupled, giving rise to a background level of neutrinos known as the [[cosmic neutrino background]] (CNB).

; Diffuse supernova neutrino background (supernova-originated):

[[Raymond Davis, Jr.]] and [[Masatoshi Koshiba]] were jointly awarded the 2002 Nobel Prize in Physics. Both conducted pioneering work on [[solar neutrino]] detection, and Koshiba's work also resulted in the first real-time observation of neutrinos from the [[SN 1987A]] supernova in the nearby [[Large Magellanic Cloud]]. These efforts marked the beginning of [[neutrino astronomy]].<ref name=Pagliarl-Vissani-etal-2009>
{{cite journal
 |last1=Pagliaroli |first1=Giulia
 |last2=Vissani |first2=Francesco
 |last3=Costantini |first3=Maria Laura
 |last4=Ianni |first4=Aldo
 |year=2009
 |title=Improved analysis of SN1987A antineutrino events
 |journal=[[Astroparticle Physics (journal)|Astroparticle Physics]]
 |volume=31 |issue=3 |pages=163–176
 |arxiv=0810.0466 |doi=10.1016/j.astropartphys.2008.12.010
 |bibcode=2009APh....31..163P |s2cid=119089069 
}}
</ref>

[[SN 1987A]] represents the only verified detection of neutrinos from a supernova. However, many stars have exploded as supernovae in the universe, leaving a theorized [[diffuse supernova neutrino background]].

== Properties and reactions ==
Neutrinos have half-integer [[Spin (physics)|spin]] ({{sfrac| 1 |2}}{{math|''ħ''}}); therefore they are [[fermion]]s. Neutrinos are [[leptons]]; therefore they are [[color charge|colorless]] fermions that cannot interact with the [[gluon]]s of the [[strong interaction|strong force]]. They have only been observed to interact through the [[weak nuclear force|weak force]], although it is assumed that they also interact gravitationally. Since they have non-zero mass, some theories permit, but do not require, neutrinos to interact magnetically; as yet there is no experimental evidence for a non-zero [[magnetic moment]] in neutrinos.<ref>{{Cite journal |last1=Giunti |first1=Carlo |last2=Studenikin |first2=Alexander |date=2015-06-16 |title=Neutrino electromagnetic interactions: A window to new physics |url=https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.87.531 |journal=Reviews of Modern Physics |volume=87 |issue=2 |pages=531–591 |doi=10.1103/RevModPhys.87.531|arxiv=1403.6344 |bibcode=2015RvMP...87..531G }}</ref>

=== Flavor, mass, and their mixing <span id="neutrino_flavor_anchor" class="anchor"></span> ===
<!-- "Neutrino flavor" redirects here -->
Weak interactions create neutrinos in one of three leptonic [[Flavor (particle physics)|flavors]]: [[electron neutrino]]s ({{math|{{SubatomicParticle|Electron neutrino}}}}), [[muon neutrino]]s ({{math|{{SubatomicParticle|Muon neutrino}}}}), or [[tau neutrino]]s ({{math|{{SubatomicParticle|Tau neutrino}}}}), associated with the corresponding charged leptons, the [[electron]] ({{math|{{SubatomicParticle|Electron}}}}), [[muon]] ({{math|{{SubatomicParticle|Muon}}}}), and [[tau (particle)|tau]] ({{math|{{SubatomicParticle|Tau}}}}), respectively.<ref name=Nakamura-Petcov-PDG-2016-νmix>
{{cite journal
 |last1=Nakamura |first1=Kengo
 |last2=Petcov   |first2=Serguey Todorov
 |year=2016
 |title=Neutrino mass, mixing, and oscillations
 |journal=Chinese Physics C
 |volume=40  |page=100001
 |url=http://pdg.lbl.gov/2016/reviews/rpp2016-rev-neutrino-mixing.pdf
 |access-date=13 December 2016  |url-status=live  |via=pdg.lbl.gov
 |archive-url=https://web.archive.org/web/20180417202541/http://pdg.lbl.gov/2016/reviews/rpp2016-rev-neutrino-mixing.pdf
 |archive-date=17 April 2018
}}</ref>

Although neutrinos were long believed to be massless, it is now known that there are three discrete neutrino masses; each neutrino flavor state is a linear combination of the three distinct mass [[Quantum state#Eigenstates_and_pure_states|eigenstates]]. From calculations based on [[cosmology|cosmological]] models, the sum of the three neutrino masses must be below {{val|0.120|u=eV/c2}}.<ref name=PDG-2024/>  Direct and independent measurements by the Karlsruhe Tritium Neutrino ([[KATRIN]]) experiment found the upper limit of the mass of the electron antineutrino as {{val|0.80|u=eV/c2}}, that is at least 5&nbsp;orders of magnitude below the next lightest [[fermion]]. This large ratio suggests the possibility that the mass-creation mechanism for neutrinos differs from that of other fermions.<ref name="KATRIN-2022-NatPhys"/> 

::{| class="wikitable" style="text-align:center;"
|+ Mass comparisons
|-
!  !! Mass {{bracket|MeV/''c''²}} !! Charge !! Source
|-
| Electron {{math|e<sup>−</sup>}} || 0.5110 || −1 {{mvar|e}} || <ref name=Greenwood-2001>{{Cite book |chapter-url=https://www.cambridge.org/core/books/an-introduction-to-nuclear-physics/leptons-and-the-electromagnetic-and-weak-interactions/C3F6EE08FBCC848FAABE60C80C9DF711 |chapter=Leptons and the electromagnetic and weak interactions |date=2001 |publisher=Cambridge University Press |isbn=978-0-521-65149-3 |editor-last=Greenwood |editor-first=D. A. |edition=2 |location=Cambridge |pages=7–18 |doi=10.1017/cbo9781139164405.006 |editor-last2=Cottingham |editor-first2=W. N. |title=An Introduction to Nuclear Physics }}</ref>
|-
| Electron neutrino {{math|ν<sub>e</sub>}} || <&nbsp;{{val|0.08|e=-6}} || &emsp;&nbsp;0&emsp; || <ref name="KATRIN-2022-NatPhys"/>
|-
| Muon {{math|μ<sup>−</sup>}} || 105.658  || −1 {{mvar|e}} || <ref name=Greenwood-2001/>
|-
| Muon neutrino {{math|ν<sub>μ</sub>}} || <&nbsp;0.17 || &emsp;&nbsp;0&emsp; || <ref name=Greenwood-2001/>
|-
| Tau {{math|τ<sup>−</sup>}} || 1777 || −1 {{mvar|e}} || <ref name=Greenwood-2001/>
|-
| Tau neutrino {{math|ν<sub>τ</sub>}} || <&nbsp;18.2 || &emsp;&nbsp;0&emsp; || <ref name=Greenwood-2001/>
|}

More formally, neutrino flavor eigenstates (creation and annihilation combinations) are not the same as the neutrino mass eigenstates (simply labeled "1", "2", and "3"). As of 2024, it is not known which of these three is the heaviest. The [[neutrino mass hierarchy]] consists of two possible configurations. In analogy with the mass hierarchy of the charged leptons, the configuration with mass&nbsp;2 being lighter than mass&nbsp;3 is conventionally called the "normal hierarchy", while in the "inverted hierarchy", the opposite would hold. Several major experimental efforts are underway to help establish which is correct.<ref name=hierarchy>
{{cite press release
 |title=Neutrino mass hierarchy
 |publisher=[[Kamioka Observatory#Hyper-Kamiokande|Hyper-Kamiokande]]
 |url=http://www.hyper-k.org/en/physics/phys-hierarchy.html
 |access-date=14 December 2016 |url-status=live
 |archive-url=https://web.archive.org/web/20170102020153/http://www.hyper-k.org/en/physics/phys-hierarchy.html
 |archive-date=2 January 2017
}}</ref>

A neutrino created in a specific flavor eigenstate is in an associated specific [[quantum superposition]] of all three mass eigenstates. The three masses differ so little that they cannot possibly be distinguished experimentally within any practical flight path. The proportion of each mass state in the pure flavor states produced has been found to depend profoundly on the flavor. The relationship between flavor and mass eigenstates is encoded in the [[PMNS matrix]]. Experiments have established moderate- to low-precision values for the elements of this matrix, with the single complex phase in the matrix being only poorly known, as of 2016.<ref name=Capozzi-Lisi-Marrone-etal-2016/>

A non-zero mass means neutrinos could have a tiny [[magnetic moment]]. If so, neutrinos would interact electromagnetically, albeit probably undetectably considering their enormous velocities. No such interaction has ever been observed.<ref>
{{cite journal
 |last1=Giunti     |first1=Carlo
 |last2=Studenikin |first2=Alexander I.
 |year=2015
 |title=Neutrino electromagnetic interactions: A window to new physics
 |journal=[[Reviews of Modern Physics]]
 |volume=87  |issue=2  |pages=531–591
 |doi=10.1103/RevModPhys.87.531  |arxiv=1403.6344
 |bibcode=2015RvMP...87..531G  |s2cid=119261485
}}
</ref>

=== Flavor oscillations ===
{{Main|Neutrino oscillation}}
Neutrinos [[Neutrino oscillation|oscillate]] between different flavors in flight. For example, an electron neutrino produced in a beta decay reaction may interact in a distant detector as a muon or tau neutrino, as defined by the flavor of the charged lepton produced in the detector. This oscillation occurs because the three mass state components of the produced flavor travel at slightly different speeds, so that their quantum mechanical [[wave packet]]s develop relative [[Phase (waves)#phase shift|phase shift]]s that change how they combine to produce a varying superposition of three flavors. Each flavor component thereby oscillates as the neutrino travels, with the flavors varying in relative strengths. The relative flavor proportions when the neutrino interacts represent the relative probabilities for that flavor of interaction to produce the corresponding flavor of charged lepton.<ref name=Grossman-Lipkin-1997/><ref name=Bilenky-2016-ν-osc/>

There are other possibilities in which neutrinos could oscillate even if they were massless: If [[Lorentz covariance|Lorentz symmetry]] were not an exact symmetry, neutrinos could experience [[Lorentz-violating neutrino oscillations|Lorentz-violating oscillations]].<ref>
{{cite journal
 |last1=Kostelecký |first1=V. Alan |author-link1=Alan Kostelecky
 |last2=Mewes |first2=Matthew
 |year=2004
 |title=Lorentz and CPT violation in neutrinos
 |journal=Physical Review D
 |volume=69 |issue=1 |article-number=016005
 |hdl=2022/18691 |s2cid=119024343
 |arxiv=hep-ph/0309025 |bibcode=2004PhRvD..69a6005A
 |doi=10.1103/PhysRevD.69.016005
}}
</ref>

=== Mikheyev–Smirnov–Wolfenstein effect ===
{{Main|Mikheyev–Smirnov–Wolfenstein effect}}
Neutrinos traveling through matter, in general, undergo a process analogous to [[Speed of light#In a medium|light traveling through a transparent material]]. This process is not directly observable because it does not produce [[ionizing radiation]], but gives rise to the [[Mikheyev–Smirnov–Wolfenstein effect]]. Only a small fraction of the neutrino's energy is transferred to the material.<ref>
{{cite web
 |title=Neutrino Oscillations
 |year=2015
 |series=Scientific background on the Nobel Prize in Physics
 |department=Class for Physics of the RSAC
 |publisher=[[Royal Swedish Academy of Sciences]]
 |website=Nobelprize.org
 |url=https://www.nobelprize.org/uploads/2017/09/advanced-physicsprize2015.pdf
 |access-date=2015-11-01 |pages=15–16
}}
</ref>

=== Antineutrinos ===
{{antimatter}}
For each neutrino, there also exists a corresponding [[antiparticle]], called an ''antineutrino'', which also has no electric charge and half-integer spin. They are distinguished from the neutrinos by having opposite signs of [[lepton number]] and opposite [[Chirality (physics)|chirality]] (and consequently opposite-sign weak isospin). As of 2016, no evidence has been found for any other difference.

So far, despite extensive and continuing searches for exceptions, in all observed leptonic processes there has never been any change in total lepton number; for example, if the total lepton number is zero in the initial state, then the final state has only matched lepton and anti-lepton pairs: electron neutrinos appear in the final state together with only positrons (anti-electrons) or electron antineutrinos, and electron antineutrinos with electrons or electron neutrinos.<ref name=FourPeaksAZ-ghostν/><ref name=HypPhys-GSU-consℓ/>

Antineutrinos are produced in nuclear beta decay together with a [[beta particle]] (in beta decay a neutron decays into a proton, electron, and antineutrino). All antineutrinos observed thus far had right-handed [[helicity (particle physics)|helicity]] (i.e., only one of the two possible spin states has ever been seen), while neutrinos were all left-handed. Nevertheless, because neutrinos have mass, their helicity is [[Frame of reference|frame]]-dependent, so particle physicists have fallen back on the frame-independent property of [[chirality]] that is closely related to helicity, and for practical purposes the same as the helicity of the ultra-relativistic neutrinos that can be observed in detectors.

Antineutrinos were first detected as a result of their interaction with protons in a large tank of water. This was installed next to a nuclear reactor as a controllable source of the antineutrinos (see ''[[Cowan–Reines neutrino experiment]]''). Researchers around the world have begun to investigate the possibility of using antineutrinos for reactor monitoring in the context of preventing the [[nuclear proliferation|proliferation of nuclear weapons]].<ref>
{{cite web
 |publisher=[[Lawrence Livermore National Laboratory]] / [[Sandia National Laboratory]]
 |year=2006
 |title=Applied Antineutrino Physics Project
 |id=LLNL-WEB-204112
 |url=https://neutrinos.llnl.gov/
 |access-date=9 April 2010
 |archive-date=21 April 2021
 |archive-url=https://web.archive.org/web/20210421150739/https://neutrinos.llnl.gov/
 |url-status=live
}}</ref><ref>
{{cite conference
 |title=Workshop
 |conference=Applied Antineutrino Physics
 |year=2007
 |place=Paris, FR
 |url=http://www.apc.univ-paris7.fr/AAP2007/
 |archive-url=https://web.archive.org/web/20071112083328/http://www.apc.univ-paris7.fr/AAP2007/
 |archive-date=2007-11-12
}}
</ref>

=== Majorana mass ===
{{See also|Seesaw mechanism}}
Because antineutrinos and neutrinos are neutral particles, it is possible that they are the same particle. Rather than conventional [[Dirac fermion]]s, neutral particles can be another type of spin&nbsp;{{sfrac| 1 |2}} particle called ''[[Majorana particle]]s'', named after the Italian physicist [[Ettore Majorana]] who first proposed the concept. For the case of neutrinos this theory has gained popularity as it can be used, in combination with the [[seesaw mechanism]], to explain why neutrino masses are so small compared to those of the other elementary particles, such as electrons or quarks. Majorana neutrinos would have the property that the neutrino and antineutrino could be distinguished only by chirality; what experiments observe as a difference between the neutrino and antineutrino could simply be due to one particle with two possible chiralities.

{{As of|2019}}, it is not known whether neutrinos are [[Majorana fermion|Majorana]] or [[Dirac fermion|Dirac]] particles. It is possible to test this property experimentally. For example, if neutrinos are indeed Majorana particles, then lepton-number violating processes such as [[neutrinoless double beta decay|neutrinoless double-beta decay]] would be allowed, while they would not if neutrinos are [[Dirac fermion|Dirac]] particles. Several experiments have been and are being conducted to search for this process, e.g. [[GERDA]],<ref name=Giunti-Kim-2007-Fundm-νν/> [[Enriched Xenon Observatory|EXO]],<ref>
{{cite journal |last=Albert |first=J B |date=June 2014 |title=Search for Majorana neutrinos with the first two years of EXO-200 data |journal=[[Nature (journal)|Nature]] |volume=510 |issue=7504 |pages=229–234 |arxiv=1402.6956 |bibcode=2014Natur.510..229T |doi=10.1038/nature13432 |issn=0028-0836 |pmid=24896189 |s2cid=2740003 |collaboration=EXO-200 Collaboration}}
</ref> [[SNO+]],<ref>
{{cite journal
 |last1=Andringa |first1=Sofia |last2=Arushanova |first2=Evelina
 |last3=Asahi |first3=Shigeo |last4=Askins |first4=Morgan
 |last5=Auty |first5=David John |last6=Back |first6=Asheley R.
 |last7=Barnard |first7=Zachariah |last8=Barros |first8=Nuno
 |last9=Beier |first9=Eugene W. |author-link9=Eugene W. Beier
 |year=2016
 |title=Current Status and Future Prospects of the SNO+ Experiment
 |journal=[[Advances in High Energy Physics]]
 |volume=2016 |pages=1–21
 |doi=10.1155/2016/6194250 |issn=1687-7357
 |arxiv=1508.05759 |s2cid=10721441
 |doi-access=free
}}
</ref> and [[CUORE]].<ref>
{{cite journal
 |first1=K. |last1=Alfonso
 |collaboration=CUORE Collaboration
 |year=2015
 |title=Search for Neutrinoless Double-Beta Decay of Te 130 with CUORE-0
 |journal=[[Physical Review Letters]]
 |volume=115 |issue=10 |article-number=102502
 |doi=10.1103/PhysRevLett.115.102502 |pmid=26382673
 |bibcode=2015PhRvL.115j2502A |arxiv=1504.02454 |s2cid=30807808
}}</ref> The [[cosmic neutrino background]] is also a probe of whether neutrinos are [[Majorana particles]], since there should be a different number of cosmic neutrinos detected in either the Dirac or Majorana case.<ref>
{{cite journal
 |last1=Long |first1=Andrew J.
 |last2=Lunardini |first2=Cecilia |author2-link=Cecilia Lunardini
 |last3=Sabancilar |first3=Eray
 |year=2014
 |title=Detecting non-relativistic cosmic neutrinos by capture on tritium: Phenomenology and physics potential
 |journal=[[Journal of Cosmology and Astroparticle Physics]]
 |volume=1408 |issue=8 |page=038
 |doi=10.1088/1475-7516/2014/08/038 |arxiv=1405.7654
 |bibcode=2014JCAP...08..038L |s2cid=119102568
}}
</ref>

=== Nuclear reactions ===
Neutrinos can interact with a nucleus, changing it to different nucleus. This process is used in radiochemical [[neutrino detector]]s. In this case, the energy levels and spin states within the target nucleus have to be taken into account to estimate the probability for an interaction. In general the interaction probability increases with the number of neutrons and protons within a nucleus.<ref name=CERN-2001-12-04-SNO>
{{cite periodical
 |title=The Sudbury Neutrino Observatory – Canada's eye on the universe
 |date=4 December 2001
 |periodical=[[CERN Courier]]
 |publisher=[[European Center for Nuclear Research]]
 |url=http://cerncourier.com/cws/article/cern/28553
 |access-date=2008-06-04
 |quote=The detector consists of a 12&nbsp;meter diameter acrylic sphere containing 1000 tonnes of heavy water ... [Solar neutrinos] are detected at SNO via the charged current process of electron neutrinos interacting with deuterons to produce two protons and an electron.
 |archive-date=25 June 2016
 |archive-url=https://web.archive.org/web/20160625002601/http://cerncourier.com/cws/article/cern/28553
 |url-status=live
}}</ref><ref name=Kelić-Zinner-Kolbe-etal-2005/>

It is very hard to uniquely identify neutrino interactions among the natural background of radioactivity. For this reason, in early experiments a special reaction channel was chosen to facilitate the identification: the interaction of an antineutrino with one of the hydrogen nuclei in the water molecules. A hydrogen nucleus is a single proton, so simultaneous nuclear interactions, which would occur within a heavier nucleus, do not need to be considered for the detection experiment. Within a cubic meter of water placed right outside a nuclear reactor, only relatively few such interactions can be recorded, but the setup is now used for measuring the reactor's plutonium production rate.

=== Induced fission and other disintegration events ===
Very much like neutrons do in [[nuclear reactor]]s, neutrinos can induce [[fission reaction]]s within heavy [[atomic nucleus|nuclei]].<ref name=Kolbe-Langanke-Fuller-2004/> So far, this reaction has not been measured in a laboratory, but is predicted to happen within stars and supernovae. The process affects the [[Abundance of the chemical elements|abundance of isotopes]] seen in the [[universe]].<ref name=Kelić-Zinner-Kolbe-etal-2005/> Neutrino-induced disintegration of [[deuterium]] nuclei has been observed in the Sudbury Neutrino Observatory, which uses a [[heavy water]] detector.<ref>{{cite journal |last1=Bellerive |first1=A |last2=Klein |first2=J.R. |last3=McDonald |first3=A.B. |last4=Noble |first4=A.J. |last5=Poon |first5=A.W.P. |title=The Sudbury Neutrino Observatory |journal=Nuclear Physics B |date=July 2016 |volume=908 |issue= |pages=30–51 |doi=10.1016/j.nuclphysb.2016.04.035 |arxiv=1602.02469 |bibcode=2016NuPhB.908...30B |s2cid=117005142 |url=https://www.sciencedirect.com/science/article/pii/S0550321316300736 |access-date=Nov 20, 2022 |archive-date=21 November 2022 |archive-url=https://web.archive.org/web/20221121044105/https://www.sciencedirect.com/science/article/pii/S0550321316300736 |url-status=live }}</ref>

=== Types ===
{| class="wikitable floatright"
|+Neutrinos in the Standard Model of elementary particles
|-
! Fermion
! Symbol
|-
!colspan="2" style="background:#ffdead;"| Generation&nbsp;1
|-
|style="background:#efefef;"| Electron neutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Electron neutrino}}}}
|-
|style="background:#efefef;"| Electron antineutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Electron antineutrino}}}}
|-
!colspan="3" style="background:#ffdead;"| Generation&nbsp;2
|-
|style="background:#efefef;"| Muon neutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Muon neutrino}}}}
|-
|style="background:#efefef;"| Muon antineutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Muon antineutrino}}}}
|-
!colspan="3" style="background:#ffdead;"| Generation&nbsp;3
|-
|style="background:#efefef;"| Tau neutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Tau neutrino}}}}
|-
|style="background:#efefef;"| Tau antineutrino
|style="text-align:center;"| {{math|{{SubatomicParticle|Tau antineutrino}}}}
|}

There are three known types (''flavors'') of neutrinos: electron neutrino {{math|{{SubatomicParticle|electron neutrino}}}}, muon neutrino {{math|{{SubatomicParticle|muon neutrino}}}}, and tau neutrino {{math|{{SubatomicParticle|tau neutrino}}}}, named after their partner leptons in the [[Standard Model]] (see table at right). The current best measurement of the number of neutrino types comes from observing the decay of the [[W and Z bosons|Z boson]]. This particle can decay into any light neutrino and its antineutrino, and the more available types of light neutrinos,{{efn|
In this context, "light neutrino" means neutrinos with less than half the mass of the Z&nbsp;boson.
}} the shorter the lifetime of the Z&nbsp;boson. Measurements of the Z lifetime have shown that three light neutrino flavors couple to the Z.<ref name=Nakamura-Petcov-PDG-2016-νmix/> The correspondence between the six [[quark]]s in the Standard Model and the six leptons, among them the three neutrinos, suggests to physicists' intuition that there should be exactly three types of neutrino.

== Research ==
There are several active research areas involving the neutrino with aspirations of finding:
* the three neutrino mass values
* the degree of [[CP violation]] in the leptonic sector (which may lead to [[Leptogenesis (physics)|leptogenesis]])
* evidence of physics which might break the Standard Model of [[particle physics]], such as [[neutrinoless double beta decay]], which would be evidence for violation of lepton number conservation.

=== Detectors near artificial neutrino sources ===
International scientific collaborations install large neutrino detectors near nuclear reactors or in neutrino beams from particle accelerators to better constrain the neutrino masses and the values for the magnitude and rates of oscillations between neutrino flavors. These experiments are thereby searching for the existence of [[CP violation]] in the neutrino sector; that is, whether or not the laws of physics treat neutrinos and antineutrinos differently.<ref name=Capozzi-Lisi-Marrone-etal-2016/>

The [[KATRIN]] experiment in Germany began to acquire data in June 2018<ref name=KATRIN-2018-06-12-pr>
{{cite press release
 |title=Die Neutrino-Waage geht in Betrieb Physik Journal
 |department=Physik News
 |website=pro-physik.de
 |date=2018-06-12 |lang=de
 |url=http://www.pro-physik.de/details/physiknews/11069692/Die_Neutrino-Waage_geht_in_Betrieb.html
 |access-date=2018-06-15
 |archive-date=2018-06-16
 |archive-url=https://web.archive.org/web/20180616053311/http://www.pro-physik.de/details/physiknews/11069692/Die_Neutrino-Waage_geht_in_Betrieb.html
}}
</ref> to determine the value of the mass of the electron neutrino, with other approaches to this problem in the planning stages.<ref name=Mertens-2016-mν/>

=== Gravitational effects ===
Despite their tiny masses, neutrinos are so numerous that their gravitational force can influence other matter in the universe.

The three known neutrino flavors are the only candidates for [[dark matter]] that are experimentally established elementary particles – specifically, they would be [[hot dark matter]]. However, the currently known neutrino types seem to be essentially ruled out as a substantial proportion of dark matter, based on observations of the [[cosmic microwave background]]. It still seems plausible that heavier, sterile neutrinos might compose [[warm dark matter]], if they exist.<ref>
{{cite journal
 |last1        = Dodelson
 |first1       = Scott
 |last2        = Widrow
 |first2       = Lawrence M.
 |year         = 1994
 |title        = Sterile neutrinos as dark matter
 |journal      = Physical Review Letters
 |volume       = 72
 |number       = 17
 |pages        = 17–20
 |arxiv        = hep-ph/9303287
 |pmid         = 10055555
 |s2cid        = 11780571
 |bibcode      = 1994PhRvL..72...17D
 |doi          = 10.1103/PhysRevLett.72.17
 |url          = https://cds.cern.ch/record/248005
 |access-date  = 21 June 2023
 |archive-date = 30 July 2020
 |archive-url  = https://web.archive.org/web/20200730101805/http://cds.cern.ch/record/248005
 |url-status   = live
}}</ref>

=== Sterile neutrino searches ===
Other efforts search for evidence of a [[sterile neutrino]] – a fourth neutrino flavor that would not interact with matter like the three known neutrino flavors.<ref>{{Cite magazine |last=McKee |first=Maggie |date=8 December 2016 |title=On a hunt for a ghost of a particle |magazine=[[Quanta Magazine]] |publisher=[[Simons Foundation]]}}</ref><ref>{{Cite arXiv |eprint=1204.5379 |class=hep-ph |first=Kevork N. |last=Abazajian |title=Light sterile neutrinos |display-authors=etal <!-- over 100 authors --> |year=2012}}
</ref><ref name="Lasserre">{{Cite journal |last=Lasserre |first=Thierry |year=2014 |title=Light sterile neutrinos in particle physics: Experimental status |journal=Physics of the Dark Universe |volume=4 |pages=81–85 |arxiv=1404.7352 |bibcode=2014PDU.....4...81L |doi=10.1016/j.dark.2014.10.001 |s2cid=118663206}}</ref><ref name="Giunti">{{Cite journal |last=Giunti |first=Carlo |year=2016 |title=Light sterile neutrinos: Status and perspectives |journal=Nuclear Physics B |volume=908 |pages=336–353 |arxiv=1512.04758 |bibcode=2016NuPhB.908..336G |doi=10.1016/j.nuclphysb.2016.01.013 |s2cid=119198173}}</ref> The possibility of sterile neutrinos is unaffected by the Z&nbsp;boson decay measurements described above: If their mass is greater than half the Z&nbsp;boson's mass, they could not be a decay product. Therefore, to be consistent with not having been detected in Z&nbsp;boson decays, heavy sterile neutrinos would need to have a mass of at least 45.6&nbsp;GeV.{{cn|date=August 2024}}

The existence of such particles is in fact hinted by experimental data from the [[LSND]] experiment. On the other hand, the currently running [[MiniBooNE]] experiment suggested that sterile neutrinos are not required to explain the experimental data,<ref>{{Cite journal |last1=Karagiorgi |first1=Georgia |last2=Aguilar-Arevalo |first2=Alexis |last3=Conrad |first3=Janet M. |author-link3=Janet Conrad |last4=Shaevitz |first4=Michael H. |last5=Whisnant |first5=Kerry |last6=Sorel |first6=Michel |last7=Barger |first7=Vernon |year=2007 |title=LeptonicCPviolation studies at MiniBooNE in the (3+2) sterile neutrino oscillation hypothesis |journal=Physical Review D |volume=75 |issue=1 |article-number=013011 |arxiv=hep-ph/0609177 |bibcode=2007PhRvD..75a3011K |doi=10.1103/PhysRevD.75.013011 |hdl=10261/9115}}</ref> although the latest research into this area is on-going and anomalies in the MiniBooNE data may allow for exotic neutrino types, including sterile neutrinos.<ref>{{Cite magazine |last=Alpert |first=Mark |year=2007 |title=Dimensional shortcuts |url=https://www.scientificamerican.com/article/dimensional-shortcuts/ |archive-url=https://web.archive.org/web/20170329063317/https://www.scientificamerican.com/article/dimensional-shortcuts/ |archive-date=2017-03-29 |access-date=2009-10-31 |magazine=[[Scientific American]]}}</ref> A re-analysis of reference electron spectra data from the [[Institut Laue-Langevin]] in 2011<ref>{{Cite journal |last1=Mueller |first1=Thomas Alexandre |last2=Lhuillier |first2=David |last3=Fallot |first3=Muriel |last4=Letourneau |first4=Alain |last5=Cormon |first5=Sandrine |last6=Fechner |first6=Maximilien |last7=Giot |first7=Lydie |last8=Lasserre |first8=Thierry |last9=Martino |first9=J. Rodriguez |last10=Mention |first10=Guillaume |last11=Porta |first11=Amanda |last12=Yermia |first12=Frédéric |display-authors=6 |year=2011 |title=Improved predictions of reactor antineutrino spectra |journal=Physical Review C |volume=83 |issue=5 |article-number=054615 |arxiv=1101.2663 |bibcode=2011PhRvC..83e4615M |doi=10.1103/PhysRevC.83.054615 |s2cid=118381633}}</ref> has also hinted at a fourth, light sterile neutrino.<ref>{{Cite journal |last1=Mention |first1=Guillaume |last2=Fechner |first2=Maximilien |last3=Lasserre |first3=Thierry |last4=Mueller |first4=Thomas Alexandre |last5=Lhuillier |first5=David |last6=Cribier |first6=Michel |last7=Letourneau |first7=Alain |year=2011 |title=Reactor antineutrino anomaly |journal=Physical Review D |volume=83 |issue=7 |article-number=073006 |arxiv=1101.2755 |bibcode=2011PhRvD..83g3006M |doi=10.1103/PhysRevD.83.073006 |s2cid=14401655}}</ref> Triggered by the 2011 findings, several experiments at very short distances from nuclear reactors have searched for sterile neutrinos since then. While most of them were able to rule out the existence of a light sterile neutrino, the combined results are ambiguous.<ref>overall
{{Cite journal |last=Schoppmann |first=Stefan |year=2021 |title=Status of anomalies and sterile neutrino searches at nuclear reactors |journal=[[Universe (journal)|Universe]] |volume=7 |issue=10 |page=360 |arxiv=2109.13541 |bibcode=2021Univ....7..360S |doi=10.3390/universe7100360 |doi-access=free}}</ref>

According to an analysis published in 2010, data from the [[Wilkinson Microwave Anisotropy Probe]] of the [[Cosmic microwave background|cosmic background radiation]] is compatible with either three or four types of neutrinos.<ref>{{Cite magazine |last=Cowen |first=Ron |date=2 February 2010 |title=New look at Big Bang radiation refines age of universe |url=https://www.wired.com/2010/02/nasa-wmap-universe-age/ |access-date=2016-11-01 |magazine=[[Wired (magazine)|Wired]] |archive-date=4 November 2016 |archive-url=https://web.archive.org/web/20161104000629/https://www.wired.com/2010/02/nasa-wmap-universe-age/ |url-status=live }}</ref>

=== Neutrinoless double-beta decay searches ===
Another hypothesis concerns "neutrinoless double-beta decay", which, if it exists, would violate lepton number conservation. Searches for this mechanism are underway but have not yet found evidence for it. If they were to, then what are now called antineutrinos could not be true antiparticles.

=== Cosmic ray neutrinos ===
[[Cosmic ray]] neutrino experiments detect neutrinos from space to study both the nature of neutrinos and the cosmic sources producing them.<ref name="IceCube">{{cite press release |title=IceCube Research Highlights |url=https://icecube.wisc.edu/science/highlights |collaboration=IceCube Collaboration |publisher=[[University of Wisconsin–Madison]] |access-date=13 December 2016 |archive-date=11 October 2016 |archive-url=https://web.archive.org/web/20161011235218/http://icecube.wisc.edu/science/highlights |url-status=live }}</ref>

=== Speed ===
{{Main|Measurements of neutrino speed}}

Before neutrinos were found to oscillate, they were generally assumed to be massless, propagating at the [[speed of light]] ({{mvar|c}}). According to the theory of [[special relativity]], the question of neutrino [[velocity]] is closely related to their [[mass]]: If neutrinos are massless, they must travel at the speed of light, and if they have mass they cannot reach the speed of light. Due to their tiny mass, the predicted speed is extremely close to the speed of light in all experiments, and current detectors are not sensitive to the expected difference.

Also, there are some [[Lorentz covariance|Lorentz-violating]] variants of [[quantum gravity]] which might allow faster-than-light neutrinos.{{citation needed|date=August 2021}} A comprehensive framework for Lorentz violations is the [[Standard-Model Extension]] (SME).

The first measurements of neutrino speed were made in the early 1980s using pulsed [[pion]] beams (produced by pulsed proton beams hitting a target). The pions decayed producing neutrinos, and the neutrino interactions observed within a time window in a detector at a distance were consistent with the speed of light. This measurement was repeated in 2007 using the [[MINOS]] detectors, which found the speed of {{val|3|ul=GeV}} neutrinos to be, at the 99% confidence level, in the range between {{val|0.999976|u={{mvar|c}}}} and {{val|1.000126|u={{mvar|c}}}}. The central value of {{val|1.000051|u={{mvar|c}}}} is higher than the speed of light but, with uncertainty taken into account, is also consistent with a velocity of exactly {{mvar|c}} or slightly less. This measurement set an upper bound on the mass of the muon neutrino at {{val|50|u=MeV}} with 99% [[confidence interval|confidence]].<ref>
{{cite journal
 |last1=Adamson |first1=Philip |last2=Andreopoulos |first2=Costas
 |last3=Arms |first3=Kregg E. |last4=Armstrong |first4=Robert
 |last5=Auty |first5=David John |last6=Avvakumov |first6=Sergei
 |last7=Ayres |first7=D.S. |last8=Baller |first8=B.
 |last9=Barish |first9=B. |last10=Barnes |first10=P.D.
 |last11=Barr |first11=G. |last12=Barrett |first12=W.L.
 |last13=Beall |first13=E. |last14=Becker |first14=B.R.
 |last15=Belias |first15=A. |last16=Bergfeld |first16=T.
 |last17=Bernstein |first17=R.H. |last18=Bhattacharya |first18=D.
 |last19=Bishai |first19=M. |last20=Blake |first20=A.
 |last21=Bock |first21=B. |last22=Bock |first22=G.J.
 |last23=Boehm |first23=J. |last24=Boehnlein |first24=D.J.
 |last25=Bogert |first25=D. |last26=Border |first26=P.M.
 |last27=Bower |first27=C. |last28=Buckley-Geer |first28=E.
 |last29=Cabrera |first29=A. |last30=Chapman |first30=J.D.
 |display-authors=6
 |year=2007
 |title=Measurement of neutrino velocity with the MINOS detectors and NuMI neutrino beam
 |journal=Physical Review D
 |volume=76 |issue=7 |article-number=072005
 |arxiv=0706.0437 |bibcode=2007PhRvD..76g2005A
 |s2cid=14358300 |doi=10.1103/PhysRevD.76.072005
}}
</ref><ref>{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=22 September 2011 |title=Tiny neutrinos may have broken cosmic speed limit |url=https://www.nytimes.com/2011/09/23/science/23speed.html |newspaper=[[The New York Times]] |quote=That group found, although with less precision, that the neutrino speeds were consistent with the speed of light. |access-date=19 February 2017 |archive-date=26 June 2018 |archive-url=https://web.archive.org/web/20180626205653/https://www.nytimes.com/2011/09/23/science/23speed.html |url-status=live }}</ref> After the detectors for the project were upgraded in 2012, MINOS refined their initial result and found agreement with the speed of light, with the difference in the arrival time of neutrinos and light of {{val|-0.0006|0.0012|u=%}}.<ref>
{{cite news
 |first=Leah
 |last=Hesla
 |date=8 June 2012
 |title=MINOS reports new measurement of neutrino velocity
 |journal=Fermilab Today
 |url=http://www.fnal.gov/pub/today/archive/archive_2012/today12-06-08.html
 |access-date=2 April 2015
 |archive-date=24 September 2015
 |archive-url=https://web.archive.org/web/20150924014225/http://www.fnal.gov/pub/today/archive/archive_2012/today12-06-08.html
 |url-status=live
}}</ref>

A similar observation was made, on a much larger scale, with supernova 1987A ([[SN 1987A]]). Antineutrinos with an energy of 10&nbsp;MeV from the supernova were detected within a time window that was consistent with the speed of light for the neutrinos. So far, all measurements of neutrino speed have been consistent with the speed of light.<ref>
{{cite journal
 |last=Stodolsky |first=Leo |author-link=Leo Stodolsky
 |year=1988
 |title=The speed of light and the speed of neutrinos
 |journal=Physics Letters B
 |volume=201 |issue=3 |pages=353–354
 |doi=10.1016/0370-2693(88)91154-9 |bibcode=1988PhLB..201..353S
}}
</ref><ref>
{{cite journal
 |first1=Andrew |last1=Cohen
 |first2=Sheldon |last2=Glashow L. |author-link2=Sheldon Glashow
 |date=28 October 2011
 |title=New constraints on neutrino velocities
 |journal=Physical Review Letters
 |volume=107 |issue=18 |article-number=181803
 |doi=10.1103/PhysRevLett.107.181803 |pmid=22107624
 |arxiv=1109.6562 |bibcode=2011PhRvL.107r1803C
 |s2cid=56198539
}}
</ref>

==== Superluminal neutrino glitch ====
{{main|Faster-than-light neutrino anomaly}}

In September&nbsp;2011, the [[OPERA experiment|OPERA collaboration]] released calculations showing velocities of 17&nbsp;GeV and 28&nbsp;GeV neutrinos exceeding the speed of light in their experiments. In November&nbsp;2011, OPERA repeated its experiment with changes so that the speed could be determined individually for each detected neutrino. The results showed the same faster-than-light speed. In February&nbsp;2012, reports came out that the results may have been caused by a loose fiber optic cable attached to one of the atomic clocks which measured the departure and arrival times of the neutrinos. An independent recreation of the experiment in the same laboratory by [[ICARUS experiment|ICARUS]] found no discernible difference between the speed of a neutrino and the speed of light.<ref name=Antonlo-Aprli-Baibusnv-etal-2012/>

=== Mass <span type="anchor" id="neutrino_mass_anchor"></span> ===
{{unsolved|physics|Can we measure the neutrino masses? Do neutrinos follow [[Fermi–Dirac statistics|Dirac]] or [[Majorana fermion|Majorana]] statistics?}}
[[File:NeutrinoMassTimeline2022.webp|thumb|upright=1.4|left|Timeline of neutrino mass measurements by different experiments<ref name=KATRIN-2022-NatPhys/>]]

The Standard Model of particle physics assumed that neutrinos are massless.<ref>
{{cite book
 |last1=Cottingham |first1=W.N.
 |last2=Greenwood |first2=D.A.
 |date=2007
 |title=An Introduction to the Standard Model of Particle Physics
 |edition=2nd
 |publisher=Cambridge University Press
|bibcode=2007ismp.book.....C
 }}
</ref> The experimentally established phenomenon of neutrino oscillation, which mixes neutrino flavor states with neutrino mass states (analogously to [[Cabibbo–Kobayashi–Maskawa matrix|CKM mixing]]), requires neutrinos to have nonzero masses.<ref>
{{cite journal
 |last1=Schechter |first1=Joseph
 |last2=Valle |first2=José W.F.
 |year=1980
 |title=Neutrino masses in SU(2) ⊗ U(1) theories
 |journal=Physical Review D
 |volume=22 |issue=9 |pages=2227–2235
 |bibcode=1980PhRvD..22.2227S |doi=10.1103/PhysRevD.22.2227
}}
</ref> Massive neutrinos were originally conceived by [[Bruno Pontecorvo]] in the 1950s. Enhancing the basic framework to accommodate their mass is straightforward by adding a right-handed Lagrangian.<ref>{{cite book |last1=Terranova |first1=Francesco |title=A Modern Primer in Particle and Nuclear Physics. |date=2021 |publisher=Oxford Univ. Press |isbn=978-0-19-284525-2}}</ref>

Providing for neutrino mass can be done in two ways, and some proposals use both:
* If, like other fundamental Standard Model fermions, mass is generated by the [[Dirac fermion|Dirac mechanism]], then the framework would require an additional right-chiral component which is an [[Special unitary group#The group SU(2)|SU(2) singlet]]. This component would have the conventional [[Yukawa interaction]]s with the neutral component of the [[Higgs boson|Higgs doublet]]; but, otherwise, would have no interactions with Standard Model particles. 
* Or, else, mass can be generated by the [[Majorana mass|Majorana mechanism]], which would require the neutrino and antineutrino to be the same particle.

A hard upper limit on the masses of neutrinos comes from [[physical cosmology|cosmology]]: the [[Big Bang]] model predicts that there is a fixed ratio between the number of neutrinos and the number of [[photon]]s in the [[cosmic microwave background radiation|cosmic microwave background]]. If the total mass of all three types of neutrinos exceeded an average of {{val|50|ul=eV/c2}} per neutrino, there would be so much mass in the universe that it would collapse.<ref>
{{cite journal
 |last1=Hut |first1=Piet |author-link1=Piet Hut
 |last2=Olive |first2=Keith A.
 |year=1979
 |title=A cosmological upper limit on the mass of heavy neutrinos
 |journal=Physics Letters B
 |volume=87 |issue=1–2 |pages=144–146
 |doi=10.1016/0370-2693(79)90039-X |bibcode=1979PhLB...87..144H
}}
</ref> This limit can be circumvented by assuming that the neutrino is unstable, but there are limits within the Standard Model that make this difficult. A much more stringent constraint comes from a careful analysis of cosmological data, such as the cosmic microwave background radiation, [[galaxy survey]]s, and the [[Lyman-alpha forest]]. Analysis of data from the WMAP microwave space telescope found that the sum of the masses of the three neutrino species must be less than {{val|0.3|u=eV/c2}}.<ref>
{{cite journal
 |last1=Goobar |first1=Ariel |last2=Hannestad |first2=Steen
 |last3=Mörtsell |first3=Edvard |last4=Tu |first4=Huitzu
 |year=2006
 |title=The neutrino mass bound from WMAP 3&nbsp;year data, the baryon acoustic peak, the SNLS supernovae and the Lyman-α forest
 |journal=Journal of Cosmology and Astroparticle Physics
 |volume=2006 |issue=6 |page=019
 |arxiv=astro-ph/0602155 |bibcode=2006JCAP...06..019G
 |s2cid=119535760 |doi=10.1088/1475-7516/2006/06/019
}}
</ref> In 2018, the [[Planck (spacecraft)|Planck collaboration]] published a stronger bound of {{val|0.11|u=eV/c2}}, which was derived by combining their CMB total intensity, polarization and gravitational lensing observations with baryon-acoustic oscillation measurements from galaxy surveys and supernova measurements from Pantheon.<ref name=planck_2018>
{{cite journal
 |author=Planck Collaboration
 |journal=Astronomy & Astrophysics
 |year=2020
 |title=Planck 2018 results. VI. Cosmological parameters
 |volume=641 |issue=A6
 |pages=A6
 |arxiv=1807.06209 |doi=10.1051/0004-6361/201833910
 |bibcode=2020A&A...641A...6P
 |s2cid=119335614
}}</ref> A 2021 reanalysis that adds redshift space distortion measurements from the SDSS-IV eBOSS survey gets an even tighter upper limit of {{val|0.09|u=eV/c2}}.<ref>
{{cite journal
 |last1=Di Valentino |first1=Eleonora
 |last2=Gariazzo |first2=Stefano
 |last3=Mena |first3=Olga
 |journal=Physical Review D
 |arxiv=2106.15267
 |doi=10.1103/PhysRevD.104.083504
 |year=2021
 |title=On the most constraining cosmological neutrino mass bounds
 |volume=104
 |issue=8
 |article-number=083504
 |s2cid=235669844
}}</ref> However, several ground-based telescopes with similarly sized error bars as Planck prefer higher values for the neutrino mass sum, indicating some tension in the data sets.<ref>
{{cite journal
 |last1=Di Valentino |first1=Eleonora
 |last2=Melchiorri |first2=Alessandro
 |arxiv=2112.02993
 |title=Neutrino Mass Bounds in the Era of Tension Cosmology
 |journal=The Astrophysical Journal Letters
 |year=2022
 |volume=931
 |issue=2
 |pages=L18
 |doi=10.3847/2041-8213/ac6ef5
 |bibcode=2022ApJ...931L..18D
 |s2cid=244909022
 |doi-access=free
}}</ref>

The Nobel prize in Physics 2015 was awarded to Takaaki Kajita and Arthur B. McDonald for their experimental discovery of neutrino oscillations, which demonstrates that neutrinos have mass.<ref>
{{cite press release
 |title=Nobel physics laureates
 |date=2015-10-06
 |publisher=The Royal Swedish Academy of Sciences
 |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.html
 |access-date=14 June 2017
 |archive-date=6 October 2015
 |archive-url=https://web.archive.org/web/20151006172856/http://www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.html
 |url-status=live
}}</ref><ref>
{{cite news
 |first=Charles |last=Day
 |date=7 October 2015
 |title=Takaaki Kajita and Arthur McDonald share 2015 Physics Nobel
 |journal=[[Physics Today]]
 |issue=10
 |issn=0031-9228 |doi=10.1063/PT.5.7208
}}
</ref>

In 1998, research results at the [[Super-Kamiokande]] neutrino detector determined that neutrinos can oscillate from one flavor to another, which requires that they must have a nonzero mass.<ref name=Fukuda-Hayakawa-Ichihara-Inoue-etal-1998/> While this shows that neutrinos have mass, the absolute neutrino mass scale is still not known. This is because neutrino oscillations are sensitive only to the difference in the squares of the masses.<ref name=Mohapatra-Antusch-Babu-Barenboim-etal-2007/> As of 2020,<ref name=Esteban-GonzlzGarc-Maltoni-Schwetz-Zhou-2020/> the best-fit value of the difference of the squares of the masses of mass eigenstates 1 and 2 is {{nowrap|{{abs|Δ''m''<sub>21</sub><sup>2</sup>}} {{=}} {{val|0.000074|u=(eV/''c''<sup>2</sup>)<sup>2</sup>}}}}, while for eigenstates 2 and 3 it is {{nowrap|{{abs|Δ''m''<sub>32</sub><sup>2</sup>}} {{=}} {{val|0.00251|u=(eV/''c''<sup>2</sup>)<sup>2</sup>}}}}. Since {{abs|Δ''m''<sub>32</sub><sup>2</sup>}} is the difference of two squared masses, at least one of them must have a value that is at least the square root of this value. Thus, there exists at least one neutrino mass eigenstate with a mass of at least {{val|0.05|u=eV/c2}}.<ref name=Amsler-Doser-Antnli-etal-2008/>

A number of efforts are under way to directly determine the absolute neutrino mass scale in laboratory experiments, especially using nuclear beta decay. Upper limits on the effective electron neutrino masses come from beta decays of tritium. The Mainz Neutrino Mass Experiment set an upper limit of {{nowrap|''m'' < {{val|2.2|u=eV/c2}}}} at 95% confidence level.<ref>
{{cite press release
 |title=The Mainz Neutrino Mass Experiment
 |url=http://www.physik.uni-mainz.de/exakt/neutrino/en_experiment.html
 |archive-url=https://web.archive.org/web/20160303220947/http://www.physik.uni-mainz.de/exakt/neutrino/en_experiment.html
 |archive-date=2016-03-03
}}</ref> Since June&nbsp;2018 the [[KATRIN]] experiment searches for a mass between {{val|0.2|u=eV/c2}} and {{val|2|u=eV/c2}} in tritium decays.<ref name=KATRIN-2018-06-12-pr/> The February 2022 upper limit is {{nowrap|''m''<sub>ν</sub> < {{val|0.8|u=eV/c2}}}} at 90% CL in combination with a previous campaign by KATRIN from 2019.<ref name="KATRIN-2022-NatPhys">
{{cite journal |last1=Aker |first1=M. |last2=Mertens |first2=S. |last3=Schlösser |first3=M. |display-authors=etal |date=February 2022 |title=Direct neutrino-mass measurement with sub-electronvolt sensitivity |journal=[[Nature Physics]] |volume=18 |issue=2 |pages=160–166 |bibcode=2022NatPh..18..160K |doi=10.1038/s41567-021-01463-1 |issn=1745-2473 |doi-access=free |collaboration=KATRIN Collaboration|hdl=1721.1/141776 |hdl-access=free |arxiv=2105.08533 }} {{ISSN|1745-2481}}&nbsp;(online)
</ref><ref>
{{cite journal
 |last=Castelvecchi
 |first=Davide
 |date=2022-02-14
 |title=How light is a neutrino? The answer is closer than ever
 |journal=[[Nature (journal)|Nature]]
 |language=en
 |doi=10.1038/d41586-022-00430-x
 |pmid=35165410
 |s2cid=246827702
 |url=https://www.nature.com/articles/d41586-022-00430-x
 |access-date=18 February 2022
 |archive-date=7 August 2023
 |archive-url=https://web.archive.org/web/20230807134612/https://www.nature.com/articles/d41586-022-00430-x
 |url-status=live
|url-access=subscription
 }}</ref>

On 31 May 2010, [[OPERA experiment|OPERA]] researchers observed the first tau neutrino candidate event in a muon neutrino beam, the first time this transformation in neutrinos had been observed, providing further evidence that they have mass.<ref name=Agafnva-Aleksndrv-Altinok-etal-2010/>

If the neutrino is a [[Majorana particle]], the mass may be calculated by finding the [[half-life]] of [[Neutrinoless double beta decay|neutrinoless double-beta decay]] of certain nuclei. The current lowest upper limit on the Majorana mass of the neutrino has been set by [[KamLAND]]-Zen: {{val|0.060|–|0.161|u=eV/c2}}.<ref>
{{cite journal
 |collaboration=KamLAND-Zen Collaboration
 |last1=Gando |first1=Azusa
 |date=11 May 2016
 |title=Search for Majorana neutrinos near the inverted mass hierarchy region with KamLAND-Zen
 |journal=Physical Review Letters
 |volume=117 |issue=8 |article-number=082503
 |doi=10.1103/PhysRevLett.117.082503 |bibcode=2016PhRvL.117h2503G
 |pmid=27588852 |arxiv=1605.02889
 |s2cid=204937469
}}</ref>

=== Chirality ===
{{main|Sterile neutrino}}
Experimental results show that within the margin of error, all produced and observed neutrinos have left-handed [[Helicity (particle physics)|helicities]] (spins antiparallel to [[Momentum|momenta]]), and all antineutrinos have right-handed helicities.<ref>
{{cite journal
 |last1=Goldhaber |first1=Maurice |author1-link=Maurice Goldhaber
 |last2=Grodzins |first2=Lee |author2-link=Lee Grodzins
 |last3=Sunyar |first3=Andrew W.
 |date=1958-01-01 |title=Helicity of neutrinos
 |journal=Physical Review
 |volume=109 |issue=3 |pages=1015–1017
 |doi=10.1103/PhysRev.109.1015 |doi-access=free
 |bibcode=1958PhRv..109.1015G
}}
</ref> In the massless limit, that means that only one of two possible chiralities is observed for either particle. These are the only chiralities included in the Standard Model of particle interactions.

It is possible that their counterparts (right-handed neutrinos and left-handed antineutrinos) simply do not exist. If they ''do'' exist, their properties are substantially different from observable neutrinos and antineutrinos.<ref>
{{cite journal
 |last1=Drewes |first1=M.
 |year=2013
 |title=The Phenomenology of Right Handed Neutrinos
 |journal=[[International Journal of Modern Physics E]]
 |volume=22 |issue=8 |pages=1330019–593
 |arxiv=1303.6912
 |bibcode=2013IJMPE..2230019D
 |doi=10.1142/S0218301313300191
 |s2cid=119161526
}}</ref> It is theorized that they are either very heavy (on the order of [[GUT scale]]—see ''[[Seesaw mechanism]]''), do not participate in weak interaction (so-called ''[[sterile neutrino]]s''), or both.

The existence of nonzero neutrino masses somewhat complicates the situation. Neutrinos are produced in weak interactions as chirality eigenstates. Chirality of a massive particle is not a constant of motion; helicity is, but the chirality operator does not share eigenstates with the helicity operator. Free neutrinos propagate as mixtures of left- and right-handed helicity states, with mixing amplitudes on the order of {{math|{{sfrac| ''m''{{sub|ν}} |''E''}}&nbsp;}}. This does not significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small. Effectively, they travel so quickly and time passes so slowly in their rest-frames that they do not have enough time to change over any observable path. For example, most solar neutrinos have energies on the order of {{val|0.1|u=MeV}} to {{val|1|u=MeV}}; consequently, the fraction of neutrinos with "wrong" helicity among them cannot exceed {{val|e=-10}}.<ref>
{{cite web
 |last=Kayser |first=Boris J. |author-link=Boris Kayser
 |year=2005
 |title=Neutrino mass, mixing, and flavor change
 |publisher=[[Particle Data Group]]
 |url=http://pdg.lbl.gov/2006/reviews/numixrpp.pdf
 |url-status=live |access-date=2007-11-25 
 |archive-url=https://web.archive.org/web/20070926103803/http://pdg.lbl.gov/2006/reviews/numixrpp.pdf
 |archive-date=26 September 2007
}}
</ref><ref>
{{cite journal
 |last1=Bilenky |first1=Samoil M.
 |last2=Giunti  |first2=Carlo
 |year=2001
 |title=Lepton numbers in the framework of neutrino mixing
 |journal=[[International Journal of Modern Physics A]]
 |volume=16  |issue=24  |pages=3931–3949
 |arxiv=hep-ph/0102320  |bibcode=2001IJMPA..16.3931B
 |s2cid=18544616  |doi=10.1142/S0217751X01004967
 |url=https://cds.cern.ch/record/489257
 |url-status=live |access-date=21 June 2023
 |archive-url=https://web.archive.org/web/20210311004309/http://cds.cern.ch/record/489257
 |archive-date=11 March 2021
}}
</ref>

=== GSI anomaly ===
{{Main|GSI anomaly}}
An unexpected series of experimental results for the rate of decay of heavy [[Highly charged ion|highly charged]] radioactive [[ion]]s circulating in a [[storage ring]] has provoked theoretical activity in an effort to find a convincing explanation. The observed phenomenon is known as the [[GSI anomaly]], as the storage ring is a facility at the [[GSI Helmholtz Centre for Heavy Ion Research]] in [[Darmstadt]], [[Germany]].

The rates of weak decay of two radioactive species with half-lives of about 40&nbsp;seconds and 200&nbsp;seconds were found to have a significant oscillatory [[modulation]], with a period of about 7&nbsp;seconds.<ref name=Kienle-Bosch-Bühler-etal-2013/> As the decay process produces an electron neutrino, some of the suggested explanations for the observed oscillation rate propose new or altered neutrino properties. Ideas related to flavor oscillation met with skepticism.<ref>
{{cite journal
 |last=Giunti |first=Carlo
 |year=2009
 |title=The GSI time anomaly: Facts and fiction
 |series=Proceedings Supplements
 |journal=Nuclear Physics B
 |volume=188 |pages=43–45
 |issn=0920-5632 |doi=10.1016/j.nuclphysbps.2009.02.009
 |arxiv=0812.1887 |bibcode=2009NuPhS.188...43G
 |citeseerx=10.1.1.250.3294 |s2cid=10196271
}}
</ref> A later proposal is based on differences between neutrino mass [[eigenstates]].<ref>
{{cite journal
 |last=Gal |first=Avraham
 |year=2016
 |title=Neutrino signals in electron-capture storage-ring experiments
 |journal=[[Symmetry (journal)|Symmetry]]
 |volume=8 |issue=6 |page=49
 |issn=2073-8994 |doi=10.3390/sym8060049
 |arxiv=1407.1789 |s2cid=14287612 |bibcode=2016Symm....8...49G
 |doi-access=free
}}
</ref>

== Sources ==
=== Artificial ===

==== Reactor neutrinos ====
Nuclear reactors are the major source of human-generated neutrinos. The majority of energy in a nuclear reactor is generated by fission (the four main fissile isotopes in nuclear reactors are {{SimpleNuclide|uranium|235|link=yes}}, {{SimpleNuclide|uranium|238|link=yes}}, {{SimpleNuclide|plutonium|239|link=yes}} and {{SimpleNuclide|plutonium|241|link=yes}}), the resultant neutron-rich daughter nuclides rapidly undergo additional beta decays, each converting one neutron to a proton and an electron and releasing an electron antineutrino. Including these subsequent decays, the average nuclear fission releases about {{val|200|u=MeV}} of energy, of which roughly 95.5% remains in the core as heat, and roughly 4.5% (or about {{val|9|u=MeV}})<ref>
{{cite book
 |title=Kay & Laby Tables of Physical and Chemical Constants
 |year=2008
 |chapter=Nuclear Fission and Fusion, and Nuclear Interactions
 |publisher=[[National Physical Laboratory (United Kingdom)|National Physical Laboratory]]
 |url=http://www.kayelaby.npl.co.uk/
 |chapter-url=http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_1.html
 |access-date=2009-06-25 
 |archive-url=https://web.archive.org/web/20060425181600/http://www.kayelaby.npl.co.uk/
 |archive-date=2006-04-25
}}
</ref> is radiated away as antineutrinos. For a typical nuclear reactor with a thermal power of {{val|4000|ul=MW}},{{efn|
Like all [[thermal power plant]]s, only about one third of the heat generated can be converted to electricity, so a {{val|4000|u=MW}} reactor would produce only {{val|1300|u=MW}} of electric power, with {{val|2700|u=MW}} being [[waste heat]].
}} the total power production from fissioning atoms is actually {{val|4185|u=MW}}, of which {{val|185|u=MW}} is radiated away as antineutrino radiation and never appears in the engineering. This is to say, {{val|185|u=MW}} of fission energy is ''lost'' from this reactor and does not appear as heat available to run turbines, since antineutrinos penetrate all building materials practically without interaction.

The antineutrino energy spectrum depends on the degree to which the fuel is burned (plutonium-239 fission antineutrinos on average have slightly more energy than those from uranium-235 fission), but in general, the ''detectable'' antineutrinos from fission have a peak energy between about 3.5 and {{val|4|u=MeV}}, with a maximum energy of about {{val|10|u=MeV}}.<ref>
{{cite journal
 |first1=Adam |last1=Bernstein
 |first2=Yifang |last2=Wang |author-link2=Wang Yifang
 |first3=Giorgio |last3=Gratta   
 |first4=Todd |last4=West    
 |date=2002
 |title=Nuclear reactor safeguards and monitoring with antineutrino detectors
 |journal=[[Journal of Applied Physics]]
 |volume=91 |issue=7 |page=4672
 |arxiv=nucl-ex/0108001 |bibcode=2002JAP....91.4672B
 |doi=10.1063/1.1452775 |s2cid=6569332
}}
</ref> There is no established experimental method to measure the flux of low-energy antineutrinos, though experiments to demonstrate the capacity of low-energy neutrino detection via the threshold-less [[Coherent elastic neutrino-nucleus scattering|CEνNS]] interaction are ongoing. Only antineutrinos with an energy above threshold of {{val|1.8|u=MeV}} can trigger [[inverse beta decay]] and thus be unambiguously identified (see ''{{section link||Detection}}'' below).

An estimated 3% of all antineutrinos from a nuclear reactor carry an energy above that threshold. Thus, an average nuclear power plant may generate over {{val|e=20}} antineutrinos per second above the threshold, but also a much larger number ({{nowrap|97% / 3% ≈ 30}} times this number) below the energy threshold; these lower-energy antineutrinos are invisible to present detector technology.

==== Accelerator neutrinos ====
{{main|Accelerator neutrinos}}
Some [[particle accelerator]]s have been used to make neutrino beams. The technique is to collide [[proton]]s with a fixed target, producing charged [[pion]]s or [[kaon]]s. These unstable particles are then magnetically focused into a long tunnel where they decay while in flight. Because of the [[Lorentz transformation|relativistic boost]] of the decaying particle, the neutrinos are produced as a beam rather than isotropically. Efforts to design an accelerator facility where neutrinos are produced through muon decays are ongoing.<ref name=Bandypdhy-Choubey-Gandhi-Goswami-etal-2009/> Such a setup is generally known as a [[Neutrino Factory|"neutrino factory"]].

==== Collider neutrinos ====
Unlike other artificial sources, colliders produce both neutrinos and anti-neutrinos of all flavors at very high energies. The first direct observation of collider neutrinos was reported in 2023 by the [[FASER experiment]] at the [[Large Hadron Collider]].<ref name="colliderneutrino">{{cite journal |last1=Worcester |first1=Elizabeth |title=The Dawn of Collider Neutrino Physics |journal=Physics |date=July 19, 2023 |volume=16 |article-number=113 |doi=10.1103/Physics.16.113 |bibcode=2023PhyOJ..16..113W |s2cid=260749625 |access-date=July 23, 2023 |url=https://physics.aps.org/articles/v16/113 |doi-access=free |archive-date=30 July 2023 |archive-url=https://web.archive.org/web/20230730054959/https://physics.aps.org/articles/v16/113 |url-status=live }}</ref>

==== Nuclear weapons ====
[[Fred Reines]] and [[Clyde Cowan]] suspected that [[nuclear weapon]]s produce very large quantities of neutrinos but concluded that short timescale of the blast would make detection impossible. They instead turned to nuclear reactors as a possible source;<ref>{{Cite web |last=Laird |first=Ian |title=Frederick Reines won a Nobel Prize for detecting the neutrino. {{!}} LANL |url=https://www.lanl.gov/media/publications/national-security-science/0325-project-poltergeist |access-date=2025-07-21 |website=Ian Laird |date=24 March 2025 |language=en}}</ref> a fission reactor was recommended as a better alternative by Los Alamos physics division leader J.M.B. Kellogg.<ref>
{{cite journal
 |first1=Reines
 |last1=Frederick
 |last2=Cowan
 |first2=Clyde L. Jr.
 |date=1997
 |title=The Reines-Cowan experiments: Detecting the poltergeist
 |journal=[[Los Alamos Science]]
 |volume=25
 |page=3
 |url=http://library.lanl.gov/cgi-bin/getfile?25-02.pdf
 |access-date=20 October 2009
 |archive-date=21 February 2013
 |archive-url=https://web.archive.org/web/20130221123519/http://library.lanl.gov/cgi-bin/getfile?25-02.pdf
 |url-status=live
}}</ref> Fission weapons produce antineutrinos (from the fission process), and fusion weapons produce both neutrinos (from the fusion process) and antineutrinos (from the initiating fission explosion).

=== Geologic ===
{{main|Geoneutrino}}
[[File:41598 2015 Article BFsrep13945 Fig1 HTML.webp|thumb|upright=1.5|AGM2015: A worldwide v̄<sub>e</sub> flux map combining [[geoneutrino]]s from natural [[Uranium-238]] and [[Thorium-232]] decay in the Earth's crust and mantle as well as manmade reactor-v̄<sub>e</sub> emitted by power reactors worldwide.]]
Neutrinos are produced together with the natural [[background radiation]]. In particular, the decay chains of {{SimpleNuclide|uranium|238|link=yes}} and {{SimpleNuclide|thorium|232|link=yes}} isotopes, as well as {{SimpleNuclide|potassium|40|link=yes}}, include beta decays which emit antineutrinos. These so-called geoneutrinos can provide valuable information on the Earth's interior. A first indication for geoneutrinos was found by the KamLAND experiment in 2005, updated results have been presented by KamLAND,<ref name=Gando-Gando-Hanakago-Ikeda-etal-2013-KamLAND/> and [[Borexino]].<ref name=Agostini-Appel-Bellini-Benziger-etal-2015/> The main background in the geoneutrino measurements are the antineutrinos coming from reactors.
[[File:Proton proton cycle.svg|upright=1.5|thumb|Solar neutrinos ([[proton–proton chain]]) in the Standard Solar Model]]

=== Atmospheric ===
Atmospheric neutrinos result from the interaction of cosmic rays with atomic nuclei in the [[Earth's atmosphere]], creating showers of particles, many of which are unstable and produce neutrinos when they decay. A collaboration of particle physicists from [[Tata Institute of Fundamental Research]] (India), [[Osaka City University]] (Japan) and [[Durham University]] (UK) recorded the first cosmic ray neutrino interaction in an underground laboratory in [[Kolar Gold Fields]] in India in 1965.<ref name=Krishnswmy-Menon-Narasmhn-etal-1971/>

=== Solar ===
{{main|Solar neutrino}}
Solar neutrinos originate from the [[nuclear fusion]] powering the [[Sun]] and other stars.
The details of the operation of the Sun are explained by the [[Standard Solar Model]]. In short: when four protons fuse to become one [[helium]] nucleus, two of them have to convert into neutrons, and each such conversion releases one electron neutrino.

The Sun sends enormous numbers of neutrinos in all directions. Each second, about 65 [[1000000000 (number)|billion]] ({{val|6.5|e=10}}) solar neutrinos pass through every square centimeter on the part of the Earth orthogonal to the direction of the Sun.<ref name=Bahcall-Serenelli-Basu-2005/> Since neutrinos are insignificantly absorbed by the mass of the Earth, the surface area on the side of the Earth opposite the Sun receives about the same number of neutrinos as the side facing the Sun.

=== Supernovae ===
{{See also|Supernova neutrinos|SuperNova Early Warning System|Diffuse supernova neutrino background}}

[[Image:Supernova-1987a.jpg|thumb|[[SN 1987A]]]]
[[Stirling Colgate|Colgate]] & White (1966)<ref name=Colgate-White-1966/> calculated that neutrinos carry away most of the gravitational energy released during the collapse of massive stars,<ref name=Colgate-White-1966/> events now categorized as [[Type Ib and Ic supernovae|Type&nbsp;Ib and Ic]] and [[Type II supernova|Type&nbsp;II]] supernovae. When such stars collapse, matter [[densities]] at the core become so high ({{val|e=17|u=kg/m3}}) that the [[degeneracy pressure|degeneracy]] of electrons is not enough to prevent protons and electrons from combining to form a neutron and an electron neutrino. [[Alfred K. Mann|Mann]] (1997)<ref name=Mann-1997-SN1987A/> found a second and more profuse neutrino source is the thermal energy (100&nbsp;billion&nbsp;[[kelvin]]s) of the newly formed neutron core, which is dissipated via the formation of neutrino–antineutrino pairs of all flavors.<ref name=Mann-1997-SN1987A/>

Colgate and White's theory of supernova neutrino production was confirmed in 1987, when neutrinos from Supernova&nbsp;1987A were detected. The water-based detectors [[Kamiokande II]] and [[Irvine–Michigan–Brookhaven (detector)|IMB]] detected 11 and 8&nbsp;antineutrinos (lepton number&nbsp;=&nbsp;&minus;1) of thermal origin,<ref name=Mann-1997-SN1987A/> respectively, while the scintillator-based [[Baksan Neutrino Observatory|Baksan]] detector found 5&nbsp;neutrinos (lepton number&nbsp;=&nbsp;+1) of either thermal or electron-capture origin, in a burst less than 13&nbsp;seconds long. The neutrino signal from the supernova arrived at Earth several hours before the arrival of the first electromagnetic radiation, as expected from the evident fact that the latter emerges along with the shock wave. The exceptionally feeble interaction with normal matter allowed the neutrinos to pass through the churning mass of the exploding star, while the electromagnetic photons were slowed.

Because neutrinos interact so little with matter, it is thought that a supernova's neutrino emissions carry information about the innermost regions of the explosion. Much of the ''visible'' light comes from the decay of radioactive elements produced by the supernova shock wave, and even light from the explosion itself is scattered by dense and turbulent gases, and thus delayed. The neutrino burst is expected to reach Earth before any electromagnetic waves, including visible light, gamma rays, or radio waves. The exact time delay of the electromagnetic waves' arrivals depends on the velocity of the shock wave and on the thickness of the outer layer of the star. For a Type&nbsp;II supernova, astronomers expect the neutrino flood to be released seconds after the stellar core collapse, while the first electromagnetic signal may emerge hours later, after the explosion shock wave has had time to reach the surface of the star. The [[SuperNova Early Warning System]] project uses a network of neutrino detectors to monitor the sky for candidate supernova events; the neutrino signal will provide a useful advance warning of a star exploding in the [[Milky Way]].<ref>{{Cite journal |last1=Antonioli |first1=Pietro |last2=Fienberg |first2=Richard Tresch |last3=Fleurot |first3=Fabrice |last4=Fukuda |first4=Yoshiyuki |last5=Fulgione |first5=Walter |last6=Habig |first6=Alec |last7=Heise |first7=Jaret |last8=McDonald |first8=Arthur B. |last9=Mills |first9=Corrinne |last10=Namba |first10=Toshio |last11=Robinson |first11=Leif J. |date=2 September 2004 |title=SNEWS: the SuperNova Early Warning System |url=https://iopscience.iop.org/article/10.1088/1367-2630/6/1/114 |journal=[[New Journal of Physics]] |volume=6 |page=114 |doi=10.1088/1367-2630/6/1/114 |arxiv=astro-ph/0406214 |bibcode=2004NJPh....6..114A |s2cid=119431247 |issn=1367-2630}}</ref>

Although neutrinos pass through the outer gases of a supernova without scattering, they provide information about the deeper supernova core with evidence that here, even neutrinos scatter to a significant extent. In a supernova core the densities are those of a neutron star (which is expected to be formed in this type of supernova),<ref>{{cite web |last=Bartusiak |first=Marcia |author-link=Marcia Bartusiak |title=The short life and violent death of Sanduleak-69 |url=http://www.marciabartusiak.com/uploads/8/5/8/9/8589314/sanduleak.pdf |website=marciabartusiak.com |access-date=2 October 2014 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304041206/http://www.marciabartusiak.com/uploads/8/5/8/9/8589314/sanduleak.pdf |url-status=live }}</ref> becoming large enough to influence the duration of the neutrino signal by delaying some neutrinos. The 13-second-long neutrino signal from SN&nbsp;1987A lasted far longer than it would take for unimpeded neutrinos to cross through the neutrino-generating core of a supernova, expected to be only 3,200&nbsp;kilometers in diameter for SN&nbsp;1987A.

The number of neutrinos counted was also consistent with a total neutrino energy of {{val|2.2|e=46|u=joules}}, which was estimated to be nearly all of the total energy of the supernova.<ref name=Pagliarl-Vissani-etal-2009/>

For an average supernova, approximately {{val|e=57}} (an [[octodecillion]]) neutrinos are released, but the actual number detected at a terrestrial detector <math>N</math> will be far smaller, at the level of
<math display="block">N \sim 10^4 \left(\frac{M}{25 \,\mathsf{kton}}\right) \left(\frac{10 \,\mathsf{kpc}}{d}\right)^2,</math>
where <math>M</math> is the mass of the detector (with e.g. [[Super Kamiokande]] having a mass of 50&nbsp;kton) and <math>d</math> is the distance to the supernova.<ref name=Beacom-Vogel-1999-ν-loc-SN/>
Hence in practice it will only be possible to detect neutrino bursts from supernovae within or nearby the [[Milky Way]] (our own galaxy). In addition to the detection of neutrinos from individual supernovae, it should also be possible to detect the [[diffuse supernova neutrino background]], which originates from all supernovae in the Universe.<ref name=Beacom-2010-diffu-SN-ν/>

=== Supernova remnants ===
The energy of supernova neutrinos ranges from a few to several tens of MeV. The sites where [[cosmic rays]] are accelerated are expected to produce neutrinos that are at least one million times more energetic, produced from turbulent gaseous environments left over by supernova explosions: [[Supernova remnant]]s. The origin of the cosmic rays was attributed to supernovas by [[Walter Baade|Baade]] and [[Fritz Zwicky|Zwicky]]; this hypothesis was refined by [[Vitaly L. Ginzburg|Ginzburg]] and [[Sergei I. Syrovatsky|Syrovatsky]] who attributed the origin to supernova remnants, and supported their claim by the crucial remark, that the cosmic ray losses of the Milky Way is compensated, if the efficiency of acceleration in supernova remnants is about 10&nbsp;percent. [[Vitaly L. Ginzburg|Ginzburg]] and Syrovatskii's hypothesis is supported by the specific mechanism of "shock wave acceleration" happening in supernova remnants, which is consistent with the original theoretical picture drawn by [[Enrico Fermi]], and is receiving support from observational data. The very high-energy neutrinos are still to be seen, but this branch of neutrino astronomy is just in its infancy. The main existing or forthcoming experiments that aim at observing very-high-energy neutrinos from our galaxy are [[Baikal Deep Underwater Neutrino Telescope|Baikal]], [[Antarctic Muon And Neutrino Detector Array|AMANDA]], [[IceCube]], [[ANTARES (telescope)|ANTARES]], [[KM3NeT|NEMO]] and [[Nestor Project|Nestor]]. Related information is provided by [[ultra-high-energy gamma ray|very-high-energy gamma ray]] observatories, such as [[VERITAS]], [[High Energy Stereoscopic System|HESS]] and [[MAGIC (telescope)|MAGIC]]. Indeed, the collisions of cosmic rays are supposed to produce charged pions, whose decay give the neutrinos, neutral pions, and gamma rays the environment of a supernova remnant, which is transparent to both types of radiation.

Still-higher-energy neutrinos, resulting from the interactions of extragalactic cosmic rays, could be observed with the [[Pierre Auger Observatory]] or with the dedicated experiment named [[Antarctic Impulsive Transient Antenna|ANITA]].

=== Big Bang ===
{{Main|Cosmic neutrino background}}
It is thought that, just like the cosmic microwave background radiation leftover from the Big Bang, there is a background of low-energy neutrinos in our Universe. In the 1980s it was proposed that these may be the explanation for the [[dark matter]] thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: They are known to exist. This idea also has serious problems.

From particle experiments, it is known that neutrinos are very light. This means that they easily move at speeds close to the [[speed of light]]. For this reason, dark matter made from neutrinos is termed "[[hot dark matter]]". The problem is that being fast moving, the neutrinos would tend to have spread out evenly in the [[universe]] before cosmological expansion made them cold enough to congregate in clumps. This would cause the part of dark matter made of neutrinos to be smeared out and unable to cause the large [[galaxy|galactic]] structures that we see.

These same galaxies and [[galaxy groups and clusters|groups of galaxies]] appear to be surrounded by dark matter that is not fast enough to escape from those galaxies. Presumably this matter provided the gravitational nucleus for [[galaxy formation and evolution|formation]]. This implies that neutrinos cannot make up a significant part of the total amount of dark matter.

From cosmological arguments, relic background neutrinos are estimated to have density of 56&nbsp;of&nbsp;each type per cubic centimeter and temperature {{val|1.9|u=K}} ({{val|1.7|e=-4|u=eV}}) if they are massless, much colder if their mass exceeds {{val|0.001|u=eV/c2}}. Although their density is quite high, they have not yet been observed in the laboratory, as their energy is below thresholds of most detection methods, and due to extremely low neutrino interaction cross-sections at sub-eV energies. In contrast, [[boron-8]] solar neutrinos—which are emitted with a higher energy—have been detected definitively despite having a space density that is lower than that of relic neutrinos by some six [[orders of magnitude]].

== Detection ==
{{Main|Neutrino detector}}

Neutrinos are extremely difficult to detect directly, as they do not carry electric charge, which means they do not ionize the materials they pass through. They however carry a [[Weak charge]], and can therefore interact with matter through the [[Weak interaction]], in both [[Charged current|charged]] and [[neutral current]] forms. However, given its short range and weak coupling, such interactions are exceedingly rare.{{cn|date=February 2025}}

Antineutrinos were first detected in the 1950s near a nuclear reactor.{{cn|date=February 2025}} Reines and Cowan used two targets containing a solution of [[cadmium chloride]] in water. Two scintillation detectors were placed next to the cadmium targets. Antineutrinos with an energy above the threshold of {{val|1.8|u=MeV}} caused charged current interactions with the protons in the water, an interaction usually known as [[inverse beta decay]], producing positrons and neutrons. This is very much like {{SubatomicParticle|Beta+}} decay, where energy is used to convert a proton into a neutron, a positron ({{SubatomicParticle|Positron}}) and an electron neutrino ({{SubatomicParticle|Electron Neutrino}}) is emitted:{{cn|date=February 2025}}
{{block indent|Energy {{math| + {{SubatomicParticle|Proton}} → {{SubatomicParticle|Neutron}} + {{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron neutrino}} }}}}

In the Cowan and Reines experiment, instead of an outgoing neutrino, an incoming antineutrino ({{SubatomicParticle|Electron Antineutrino}}) from a nuclear reactor interacts with a proton:

{{block indent|Energy (>&nbsp;{{val|1.8|u=MeV}}) {{math| + {{SubatomicParticle|Proton}} + {{SubatomicParticle|Electron antineutrino}} → {{SubatomicParticle|Neutron}} + {{SubatomicParticle|Positron}} }}}}

The resulting positron annihilation with electrons in the detector material created photons with an energy of about {{val|0.5|u=MeV}}. Pairs of photons in coincidence could be detected by the two scintillation detectors above and below the target. The neutrons were captured by cadmium nuclei resulting in gamma rays of about {{val|8|u=MeV}} that were detected a few microseconds after the photons from a positron annihilation event.

Since then, various detection methods have been used. [[Super Kamiokande]] is a large volume of water surrounded by [[photomultiplier tube]]s that watch for the [[Cherenkov radiation]] emitted when an incoming neutrino creates an electron or muon in the water. The Sudbury Neutrino Observatory is similar, but used [[heavy water]] as the detecting medium, which uses the same effects, but also allows the additional reaction any-flavor neutrino photo-dissociation of deuterium, resulting in a free neutron which is then detected from gamma radiation after chlorine-capture. Other detectors have consisted of large volumes of [[chlorine]] or [[gallium]] which are periodically checked for excesses of [[argon]] or [[germanium]], respectively, which are created by electron-neutrinos interacting with the original substance. MINOS used a solid plastic [[scintillator]] coupled to photomultiplier tubes, while Borexino uses a liquid [[pseudocumene]] scintillator also watched by photomultiplier tubes and the [[NOνA]] detector uses liquid scintillator watched by [[avalanche photodiode]]s. The [[IceCube Neutrino Observatory]] uses {{val|1|u=km3}} of the [[Antarctic ice sheet]] near the [[South Pole]] with photomultiplier tubes distributed throughout the volume. 

More exotically, some experiments (such as [[The COHERENT Collaboration|COHERENT]] and [[CONUS-Experiment|CONUS]]) leverage the neutral current interaction of neutrinos with a whole nucleus, the [[Coherent elastic neutrino-nucleus scattering|CEνNS]] interaction, to detect neutrinos below the threshold of inverse beta decay. These experiments, which overwhelmingly use crystal-based detectors very similar to the solid-state detectors in use for [[direct detection of dark matter]] experiments, are some of the most sensitive particle detectors in modern physics, boasting thresholds as low as 20&nbsp;eV<ref>{{Cite journal |last1=Strauss |first1=R. |last2=Rothe |first2=J. |last3=Angloher |first3=G. |last4=Bento |first4=A. |last5=Gütlein |first5=A. |last6=Hauff |first6=D. |last7=Kluck |first7=H. |last8=Mancuso |first8=M. |last9=Oberauer |first9=L. |last10=Petricca |first10=F. |last11=Pröbst |first11=F. |last12=Schieck |first12=J. |last13=Schönert |first13=S. |last14=Seidel |first14=W. |last15=Stodolsky |first15=L. |date=August 2017 |title=The ''ν''-cleus experiment: a gram-scale fiducial-volume cryogenic detector for the first detection of coherent neutrino–nucleus scattering |journal=The European Physical Journal C |language=en |volume=77 |issue=8 |page=506 |doi=10.1140/epjc/s10052-017-5068-2 |doi-access=free |arxiv=1704.04320 |bibcode=2017EPJC...77..506S }}</ref> deposited in the detector. This is necessary as the heavier nuclei, selected for the high probability of interaction, will retain very little of the energy in an elastic scattering, being much more massive than the neutrino.

Other ways neutrinos might affect their environment, such as the [[MSW effect]], do not produce traceable radiation, and are not predicted to be detectable.

== Scientific interest ==
Neutrinos' low mass and neutral charge mean they interact exceedingly weakly with other particles and fields. This feature of weak interaction interests scientists because it means neutrinos can be used to probe environments that other radiation (such as light or radio waves) cannot penetrate.

Using neutrinos as a probe was first proposed in the mid-20th&nbsp;century as a way to detect conditions at the core of the Sun. The solar core cannot be imaged directly because electromagnetic radiation (such as light) is diffused by the great amount and density of matter surrounding the core. On the other hand, neutrinos pass through the Sun with few interactions. Whereas photons emitted from the solar core may require {{val|40,000|u=years}} to diffuse to the outer layers of the Sun, neutrinos generated in stellar fusion reactions at the core cross this distance practically unimpeded at nearly the speed of light.<ref name=Bahcall-1989-ν-astroph/><ref name=Davis-2003-NobLects/>

Neutrinos are also useful for probing [[Neutrino astronomy|astrophysical]] sources beyond the Solar System because they are the only known particles that are not significantly [[attenuation|attenuated]] by their travel through the interstellar medium. Optical photons can be obscured or diffused by dust, gas, and background radiation. High-energy cosmic rays, in the form of swift protons and atomic nuclei, are unable to travel more than about 100&nbsp;[[megaparsec]]s due to the [[Greisen–Zatsepin–Kuzmin limit]] (GZK cutoff). Neutrinos, in contrast, can travel even greater distances barely attenuated.

The galactic core of the Milky Way is fully obscured by dense gas and numerous bright objects. Neutrinos produced in the galactic core might be measurable by Earth-based [[neutrino telescope]]s.<ref name=Close-2012-ν/>

Another important use of the neutrino is in the observation of [[supernova]]e, the explosions that end the lives of highly massive stars. The core collapse phase of a supernova is an extremely dense and energetic event. It is so dense that no known particles are able to escape the advancing core front except for neutrinos. Consequently, supernovae are known to release approximately 99% of their [[radiant energy]] in a short (10-second) burst of neutrinos.<ref name=Minkel-2009-07-17-ν-gunsmoke/> These neutrinos are a very useful probe for core collapse studies.

The rest mass of the neutrino is an important test of cosmological and astrophysical theories. The neutrino's significance in probing cosmological phenomena is as great as any other method, and is thus a major focus of study in astrophysical communities.<ref name=Gelmini-Kusenko-Weiler-2010/>

The study of neutrinos is important in [[particle physics]] because neutrinos typically have the lowest rest mass among massive particles (i.e. the lowest non-zero rest mass, i.e. excluding the zero rest mass of photons and gluons), and hence are examples of the lowest-energy massive particles theorized in extensions of the Standard Model of particle physics.

In November&nbsp;2012, American scientists used a particle accelerator to send a coherent neutrino message through 780&nbsp;feet of rock. This marks the first use of neutrinos for communication, and future research may permit binary neutrino messages to be sent immense distances through even the densest materials, such as the Earth's core.<ref name=Stancil-Adamson-Alania-etal-2012/>

In July&nbsp;2018, the IceCube Neutrino Observatory announced that they have traced an extremely-high-energy neutrino that hit their Antarctica-based research station in September&nbsp;2017 back to its point of origin in the blazar [[TXS 0506+056]] located 3.7&nbsp;billion [[light-year]]s away in the direction of the constellation [[Orion (constellation)|Orion]]. This is the first time that a [[neutrino detector]] has been used to locate an object in space and that a source of cosmic rays has been identified.<ref name=Overbye-2018-07-12-Antc-ν/><ref name=Sample-2018-07-12-ν-Antc/><ref name=Halton-2018-07-12-ν-src/>

In November&nbsp;2022, the IceCube Neutrino Observatory found evidence of high-energy neutrino emission from NGC 1068, also known as [[Messier 77]], an active galaxy in the constellation Cetus and one of the most familiar and well-studied galaxies to date.<ref>{{cite press release |url=https://icecube.wisc.edu/news/press-releases/2022/11/icecube-neutrinos-give-us-first-glimpse-into-the-inner-depths-of-an-active-galaxy/ |title=IceCube neutrinos give us first glimpse into the inner depths of an active galaxy |work=[[IceCube Neutrino Observatory]] |publisher=[[University of Wisconsin–Madison]] |date=3 November 2022 |access-date=22 November 2022 |archive-date=22 September 2023 |archive-url=https://web.archive.org/web/20230922052959/https://icecube.wisc.edu/news/press-releases/2022/11/icecube-neutrinos-give-us-first-glimpse-into-the-inner-depths-of-an-active-galaxy/ |url-status=live }}</ref>

In June 2023, astronomers reported using a new technique to detect, for the first time, the release of neutrinos from the [[galactic plane]] of the Milky Way [[galaxy]].<ref name="NYT-20230629">{{cite news |last=Chang |first=Kenneth |title=Neutrinos Build a Ghostly Map of the Milky Way - Astronomers for the first time detected neutrinos that originated within our local galaxy using a new technique. |url=https://www.nytimes.com/2023/06/29/science/neutrinos-milky-way-map.html |work=[[The New York Times]] |date=29 June 2023 |url-status=live |archive-url=https://archive.today/20230629182106/https://www.nytimes.com/2023/06/29/science/neutrinos-milky-way-map.html |archive-date=29 June 2023 |access-date=30 June 2023 }}</ref><ref name="SCI-20230629">{{cite journal |author=IceCube Collaboration |title=Observation of high-energy neutrinos from the Galactic plane |url=https://www.science.org/doi/10.1126/science.adc9818 |date=29 June 2023 |journal=[[Science (journal)|Science]] |volume=380 |issue=6652 |pages=1338–1343 |doi=10.1126/science.adc9818 |pmid=37384687 |arxiv=2307.04427 |hdl=2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/360407 |s2cid=259287623 |url-status=live |archive-url=https://archive.today/20230630042539/https://www.science.org/doi/10.1126/science.adc9818 |archive-date=30 June 2023 |access-date=30 June 2023 }}</ref>

== See also ==
* {{anl|List of neutrino experiments}}
* {{anl|Multi-messenger astronomy}}
* {{anl|Neutrino oscillation}}
* {{anl|Neutrino astronomy}}
* {{anl|Pontecorvo–Maki–Nakagawa–Sakata matrix}}

== Notes ==
{{notelist}}

== References ==
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 |last17=Nakayama |first17=S. |last18=Okada |first18=A.
 |last19=Oketa |first19=M. |last20=Okumura |first20=K.
 |last21=Ota |first21=M. |last22=Sakurai |first22=N.
 |last23=Shiozawa |first23=M. |last24=Suzuki |first24=Y.
 |last25=Takeuchi |first25=Y. |last26=Totsuka |first26=Y.
 |last27=Yamada |first27=S. |last28=Earl |first28=M.
 |last29=Habig |first29=A. |last30=Hong |first30=J.T.
 |display-authors=6
 |year=1998
 |title=Measurements of the solar neutrino flux from Super-Kamiokande's first 300&nbsp;days
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 |volume=81 |issue=6 |pages=1158–1162
 |arxiv=hep-ex/9805021 |bibcode=1998PhRvL..81.1158F
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}}
</ref>

<ref name=Gando-Gando-Hanakago-Ikeda-etal-2013-KamLAND>
{{cite journal
 |collaboration=KamLAND Collaboration
 |last1=Gando |first1=Azusa |last2=Gando |first2=Yoshihito
 |last3=Hanakago |first3=H. |last4=Ikeda |first4=Haruo
 |last5=Inoue |first5=Kunio |last6=Ishidoshiro |first6=Koji
 |last7=Ishikawa |first7=H. |last8=Koga |first8=Masayuki <!--
R. Matsuda, S. Matsuda, Tadao Mitsui, D. Motoki, Kengo Nakamura, A. Obata, A. Oki, Y. Oki, M. Otani, Itaru Shimizu, Jyunpei Shirai, Atsuto Suzuki, Y. Takemoto, Kyoko Tamae, Kota Ueshima, Hiroko Watanabe, B. D. Xu, S. Yamada, Y. Yamauchi, H. Yoshida, A. Kozlov, S. Yoshida, A. Piepke, T. I. Banks, B. K. Fujikawa, K. Han, T. O'Donnell, B. E. Berger, J. G. Learned, S. Matsuno, M. Sakai, Y. Efremenko, H. J. Karwowski, D. M. Markoff, W. Tornow, J. A. Detwiler, S. Enomoto, M. P. Decowski
-->
 |display-authors=6
 |date=2013-08-02 |title=Reactor on-off antineutrino measurement with KamLAND
 |journal=Physical Review D
 |volume=88 |issue=3 |article-number=033001
 |doi=10.1103/PhysRevD.88.033001 |bibcode=2013PhRvD..88c3001G 
 |arxiv=1303.4667 |s2cid=55754667
}}
</ref>

<ref name=Gelmini-Kusenko-Weiler-2010>
{{cite magazine |last1=Gelmini |first1=Graciela B. |author-link1=Graciela B. Gelmini |last2=Kusenko |first2=Alexander |author2-link=Alexander Kusenko |last3=Weiler |first3=Thomas J. |date=May 2010 |title=Through Neutrino Eyes |magazine=Scientific American |volume=302 |issue=5 |pages=38–45 |bibcode=2010SciAm.302e..38G |doi=10.1038/scientificamerican0510-38 |url=http://www.scientificamerican.com/article.cfm?id=through-neutrino-eyes |access-date=8 October 2011 |archive-date=3 December 2013 |archive-url=https://web.archive.org/web/20131203020822/http://www.scientificamerican.com/article.cfm?id=through-neutrino-eyes |url-status=live }}
</ref>

<ref name=Giunti-Kim-2007-Fundm-νν>
{{cite book
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 |last1=Giunti
 |first2=Chung W.
 |last2=Kim
 |year=2007
 |title=Fundamentals of Neutrino Physics and Astrophysics
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 |page=255
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 |isbn=978-0-19-850871-7
 |access-date=18 November 2020
 |archive-date=14 September 2023
 |archive-url=https://web.archive.org/web/20230914152936/https://books.google.com/books?id=SdAcSwTR0CgC&q=majorana%20neutrino%20helicity&pg=PA255
 |url-status=live
}}
</ref>

<ref name=Halton-2018-07-12-ν-src>
{{cite web
 |first=Mary
 |last=Halton
 |date=12 July 2018
 |title=Source of cosmic 'ghost' particle revealed
 |work=[[BBC News]]
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 |access-date=4 December 2019
 |archive-date=13 July 2019
 |archive-url=https://web.archive.org/web/20190713212712/https://www.bbc.com/news/science-environment-44786125
 |url-status=live
}}
</ref>

<ref name=Kelić-Zinner-Kolbe-etal-2005>
{{cite journal
 |last1=Kelić |first1=Aleksandra |last2=Zinner |first2=Nikolaj T.
 |last3=Kolbe |first3=Edwin |last4=Langanke |first4=Karlheinz
 |last5=Schmidt |first5=Karl-Heinz
 |year=2005
 |title=Cross sections and fragment distributions from neutrino-induced fission on r-process nuclei
 |journal=Physics Letters B
 |volume=616 |issue=1–2 |pages=48–58
 |doi=10.1016/j.physletb.2005.04.074 |doi-access=free
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}}
</ref>

<ref name=Kienle-Bosch-Bühler-etal-2013>
{{cite journal
 |last1=Kienle |first1=Paul |last2=Bosch |first2=Fritz
 |last3=Bühler |first3=Paul |last4=Faestermann |first4=Thomas
 |last5=Litvinov |first5=Yuri A. |last6=Sanjari |first6=M. Shahab
 |last7=Shubina |first7=D.B. |last8=Winckler |first8=N.
 |last9=Atanasov |first9=D. |last10=Geissel |first10=H.
 |last11=Ivanova |first11=V. |last12=Yan |first12=X.L.
 |last13=Boutin |first13=D. |last14=Brandau |first14=C.
 |last15=Dillmann |first15=I. |last16=Dimopoulou |first16=Ch.
 |last17=Hess |first17=R. |last18=Hillebrand |first18=P.-M.
 |last19=Izumikawa |first19=T. |last20=Knöbel |first20=R.
 |last21=Kurcewicz |first21=J. |last22=Kuzminchuk |first22=N.
 |last23=Lestinsky |first23=M. |last24=Litvinov |first24=Sergey A.
 |last25=Ma |first25=X.W. |last26=Maier |first26=L.
 |last27=Mazzocco |first27=M. |last28=Mukha |first28=I.
 |last29=Nociforo |first29=C. |last30=Nolden |first30=F.
 |last31=Scheidenberger |first31=Ch. |last32=Spillmann |first32=U.
 |last33=Steck |first33=M. |last34=Stöhlker |first34=Th.
 |last35=Sun |first35=B.H. |last36=Suzaki |first36=F.
 |last37=Suzuki |first37=T. |last38=Torilov |first38=S.Yu.
 |last39=Trassinelli |first39=M. |last40=Tu |first40=X.L.
 |last41=Wang |first41=M. |last42=Weick |first42=H.
 |last43=Winters |first43=D.F.A. |last44=Winters |first44=N.
 |last45=Woods |first45=P.J. |last46=Yamaguchi |first46=T.
 |last47=Zhang |first47=G.L. |last48=Ohtsubo |first48=T.
 |display-authors=6
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 |journal=Physics Letters B
 |volume=726 |issue=4–5 |pages=638–645
 |year=2013
 |issn=0370-2693 |doi=10.1016/j.physletb.2013.09.033
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}}
</ref>

<ref name=Kolbe-Langanke-Fuller-2004>
{{cite journal
 |last1=Kolbe |first1=Edwin
 |last2=Langanke |first2=Karlheinz
 |last3=Fuller |first3=George M.
 |year=2004
 |title=Neutrino-Induced Fission of Neutron-Rich Nuclei
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 |volume=92 |issue=11 |article-number=111101
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 |s2cid=32443855
 |url=https://cds.cern.ch/record/638993
}}
</ref>

<ref name=Krishnswmy-Menon-Narasmhn-etal-1971>
{{cite journal
 |last1=Krishnaswamy |first1=M.R. 
 |last2=Menon |first2=Mambillikalathil Govind Kumar |author-link2=M. G. K. Menon
 |last3=Narasimhan |first3=V.S.   <!-- maybe  Mudumbai Seshachalu? --> 
 |last4=Hinotani |first4=Kensaku |last5=Ito |first5=Nobuo
 |last6=Miyake |first6=Saburo |last7=Osborne |first7=John L.
 |last8=Parsons |first8=A.J. |last9=Wolfendale |first9=Arnold W. 
 |display-authors=6
 |date=6 July 1971
 |series=The Kolar Gold Fields neutrino experiment
 |title=II. Atmospheric muons at a depth of 7000&nbsp;hg&nbsp;cm² (Kolar)
 |journal=[[Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences]]
 |volume=323 |issue=1555 |pages=511–522
 |doi=10.1098/rspa.1971.0120 |bibcode=1971RSPSA.323..511K
 |s2cid=120583187 |jstor=78071 
}}
</ref>

<ref name=Mann-1997-SN1987A>
{{cite book
 |last=Mann |first=Alfred K. |author-link=Alfred K. Mann
 |date=1997
 |title=Shadow of a star: The neutrino story of Supernova&nbsp;1987A
 |page=122
 |publisher=[[W. H. Freeman]]
 |isbn=978-0-7167-3097-2
 |url=http://www.whfreeman.com/GeneralReaders/book.asp?disc=TRAD&id_product=1058001008&@id_course=1058000240
 |access-date=2007-11-20  
 |archive-url=https://web.archive.org/web/20080505003516/http://www.whfreeman.com/GeneralReaders/book.asp?disc=TRAD&id_product=1058001008&@id_course=1058000240
 |archive-date=2008-05-05 |df=ymd-all
}}
</ref>

<ref name=Minkel-2009-07-17-ν-gunsmoke>
{{cite journal
 |last=Minkel
 |first=J.R.
 |date=2009-07-17
 |title=Supernova starting gun: Neutrinos
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 |url=http://focus.aps.org/story/v24/st4
 |access-date=2012-04-05
 |archive-date=26 September 2011
 |archive-url=https://web.archive.org/web/20110926221939/http://focus.aps.org/story/v24/st4
 |url-status=live
}}
</ref>

<ref name=Mohapatra-Antusch-Babu-Barenboim-etal-2007>
{{cite journal |last1=Mohapatra |first1=Rabindra N. |author-link1=Rabindra Mohapatra |last2=Antusch |first2=Stefan |last3=Babu |first3=Kaladi S. |author-link3=K. S. Babu |last4=Barenboim |first4=Gabriela |last5=Chen |first5=Mu-Chun |last6=de Gouvêa |first6=André |last7=de Holanda |first7=Pedro Cunha |last8=Dutta |first8=Bhaskar |last9=Grossman |first9=Yuval |last10=Joshipura |first10=Anjan S. |last11=Kayser |first11=Boris J. |last12=Kersten |first12=Joern |last13=Keum |first13=Yong-Yeon |last14=King |first14=Stephen F. |last15=Langacker |first15=Paul |last16=Lindner |first16=Manfred |last17=Loinaz |first17=Will A. |last18=Masina |first18=Isabella |last19=Mocioiu |first19=Irina |last20=Mohanty |first20=Subhendra |last21=Murayama |first21=Hitoshi |last22=Pascoli |first22=Silvia |last23=Petcov |first23=Serguey Todorov |last24=Pilaftsis |first24=A. |last25=Ramond |first25=P. |last26=Ratz |first26=M. |last27=Rodejohann |first27=W. |last28=Shrock |first28=R. |last29=Takeuchi |first29=T. |last30=Underwood |first30=T. |display-authors=6 |year=2007 |title=Theory of neutrinos |series=White Paper |journal=[[Reports on Progress in Physics]] |volume=70 |issue=11 |pages=1757–1867 |arxiv=hep-ph/0510213 |bibcode=2007RPPh...70.1757M |s2cid=119092531 |doi=10.1088/0034-4885/70/11/R02 |hdl=10550/43454 |url=https://digital.library.unt.edu/ark:/67531/metadc878201/ |access-date=13 August 2018 |archive-date=13 August 2018 |archive-url=https://web.archive.org/web/20180813111422/https://digital.library.unt.edu/ark:/67531/metadc878201/ |url-status=live }}
</ref>

<ref name=Overbye-2018-07-12-Antc-ν>
{{cite news
 |last=Overbye
 |first=Dennis
 |title=It came from a black hole and landed in Antarctica
 |quote=For the first time, astronomers followed cosmic neutrinos into the fire-spitting heart of a supermassive blazar
 |url=https://www.nytimes.com/2018/07/12/science/space-neutrinos-blazar.html
 |date=12 July 2018
 |newspaper=[[The New York Times]]
 |access-date=13 July 2018
 |archive-date=14 May 2019
 |archive-url=https://web.archive.org/web/20190514085628/https://www.nytimes.com/2018/07/12/science/space-neutrinos-blazar.html
 |url-status=live
}}
</ref>

<ref name=Sample-2018-07-12-ν-Antc>
{{cite news
 |last=Sample
 |first=Ian
 |date=12 July 2018
 |title=Neutrino that struck Antarctica traced to galaxy 3.7&nbsp;bn light years away
 |newspaper=[[The Guardian]]
 |url=https://www.theguardian.com/science/2018/jul/12/neutrino-that-struck-antarctica-traced-to-galaxy-37bn-light-years-away
 |access-date=13 July 2018
 |archive-date=15 August 2019
 |archive-url=https://web.archive.org/web/20190815155308/https://www.theguardian.com/science/2018/jul/12/neutrino-that-struck-antarctica-traced-to-galaxy-37bn-light-years-away
 |url-status=live
}}
</ref>

<ref name=Stancil-Adamson-Alania-etal-2012>
{{cite journal |last1=Stancil |first1=Daniel D. |last2=Adamson |first2=Philip |last3=Alania |first3=Marcos |last4=Aliaga Soplin |first4=Leo |last5=Andrews |first5=Michael P. |last6=Araujo del Castillo |first6=Carmen |last7=Bagby |first7=L. |last8=Bazo Alba |first8=J.L. |last9=Bodek |first9=A. |last10=Boehnlein |first10=D. |last11=Bradford |first11=R. |last12=Brooks |first12=W.K. |last13=Budd |first13=H. |last14=Butkevich |first14=A. |last15=Caicedo |first15=D.A.M. |last16=Capista |first16=D.P. |last17=Castromonte |first17=C.M. |last18=Chamorro |first18=A. |last19=Charlton |first19=E. |last20=Christy |first20=M.E. |last21=Chvojka |first21=J. |last22=Conrow |first22=P.D. |last23=Danko |first23=I. |last24=Day |first24=M. |last25=Devan |first25=J. |last26=Downey |first26=J.M. |last27=Dytman |first27=S. A. |last28=Eberly |first28=B. |last29=Fein |first29=J.R. |last30=Felix |first30=J. |display-authors=6 |date=2012 |title=Demonstration of communication using neutrinos |journal=[[Modern Physics Letters A]] |volume=27 |issue=12 |pages=1250077–1–1250077–10 |bibcode=2012MPLA...2750077S |url=http://lss.fnal.gov/archive/2012/pub/fermilab-pub-12-073-e.pdf |doi=10.1142/S0217732312500770 |arxiv=1203.2847 |s2cid=119237711 <!-- ----- --> |access-date=27 January 2015 |archive-date=13 February 2015 |archive-url=https://web.archive.org/web/20150213035926/http://lss.fnal.gov/archive/2012/pub/fermilab-pub-12-073-e.pdf |url-status=live }}

* {{cite magazine |author=Rebecca Boyle |date=15 March 2012 |title=For the First Time, a Message Sent With Neutrinos |magazine=[[Popular Science]] |url=https://www.popsci.com/science/article/2012-03/first-time-neutrinos-send-message-through-bedrock }}
</ref>

}} <!-- end of "refs=" -->

== Bibliography ==
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* {{cite journal
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* {{cite journal
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* {{cite book
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* {{cite magazine
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* {{cite book
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* {{cite journal
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* {{cite book
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* {{cite book
 |last=Jayawardhana |first=Ray
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}}
* {{cite book
 |last=Perkins |first=Donald H. |author-link=Donald Hill Perkins
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* {{cite book
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 |year=1995
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* {{cite web
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* {{cite book
 |last=Schopper |first=Herwig F. |author-link=Herwig Schopper
 |year=1966
 |title=Weak Interactions and Nuclear Beta Decay
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* {{cite journal
 |last1=Tammann |first1=Gustav A.A. <!-- Gustav Alfred Andreas --> |author-link=Gustav Andreas Tammann
 |last2=Thielemann |first2=Friedrich-Karl |author-link2=Friedrich-Karl Thielemann
 |last3=Trautmann |first3=Dirk
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* {{cite book
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}}
* {{cite book
 |last=Tomonaga |first=Sin-itiro  <!-- lower case "itiro" --> |author-link=Shin'ichirō Tomonaga
 |year=1997
 |title=The Story of Spin
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 |translator=Oka, Takeshi 
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}}
* {{cite book
 |last=Zuber |first=Kai
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 |publisher=[[IOP Publishing]]
 |doi=10.1201/9781315195612
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 |url=https://www.taylorfrancis.com/books/oa-mono/10.1201/9781315195612/neutrino-physics-kai-zuber
}}
{{refend}}

== External links ==
{{Wiktionary|neutrino}}
* {{cite web
 |url=http://www.ps.uci.edu/~superk/neutrino.html
 |title=What's a Neutrino?
 |first=Dave |last=Casper
 |publisher=[[University of California, Irvine]]
 |access-date=2009-10-31
 |archive-date=2010-09-25
 |archive-url=https://web.archive.org/web/20100925101827/http://www.ps.uci.edu/~superk/neutrino.html
}}
* {{cite web
 |url=https://www.nu.to.infn.it/
 |title=Neutrino unbound
 |website=On-line review and e-archive on Neutrino Physics and Astrophysics
 |first1=Stefano |last1=Gariazzo
 |first2=Carlo |last2=Giunti
 |first3=Marco |last3=Laveder
}}
* {{cite AV media
 |url=https://www.pbs.org/wgbh/nova/neutrino/
 |series=Nova
 |title=The Ghost Particle
 |medium=video documentary
 |publisher=WGBH
 |place=Boston, MA
 |first=David |last=Sington |author-link=David Sington
}}
* {{cite web
 |url=https://neutrinos.fnal.gov/
 |title=All Things Neutrino
 |publisher=[[Fermilab]]
}}
* {{cite news
 |url=https://www.newscientist.com/article/dn13414-universe-submerged-in-a-sea-of-chilled-neutrinos.html
 |title=Universe submerged in a sea of chilled neutrinos
 |magazine=[[New Scientist]]
 |date=5 March 2008
 |first=Stephen |last=Battersby |author-link=Stephen Battersby (science journalist)
}}
* {{cite web
 |url=http://www.hep.anl.gov/ndk/hypertext/
 |title=The neutrino oscillation industry
 |first=Maury C. |last=Goodman |author-link=Maury C. Goodman
 |publisher=[[Argonne National Laboratory]]
}}
* {{cite journal
 |first1=Hans Volker |last1=Klapdor-Kleingrothaus |author-link1=Hans Volker Klapdor-Kleingrothaus
 |first2=Irina V. |last2=Krivosheina
 |first3=Alexander |last3=Dietz
 |first4=Oleg |last4=Chkvorets
 |date=2007-09-27
 |title=Search for neutrinoless double beta decay with enriched <sup>76</sup>Ge in Gran Sasso 1990–2003
 |journal=Physics Letters B
 |url=http://www.mpi-hd.mpg.de/non_acc/POSITIVE-EVID/NEW-2004/PL586-2004.pdf
 |archive-url=https://web.archive.org/web/20070927012833/http://www.mpi-hd.mpg.de/non_acc/POSITIVE-EVID/NEW-2004/PL586-2004.pdf
 |archive-date=2007-09-27
}}
* {{cite news
 |first=George |last=Johnson |author-link=George Johnson (writer)
 |date=2002-04-28 |title=Cosmic weight gain: A wispy particle bulks up
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/2002/04/28/weekinreview/ideas-trends-cosmic-weight-gain-a-wispy-particle-bulks-up.html?pagewanted=all&src=pm
}}
* {{cite news
 |first=Paul |last=Rincon |author-link=Paul Rincon
 |date=22 June 2010
 |title=Neutrino 'ghost particle' sized up by astronomers
 |website=[[BBC News Online|BBC News]]
 |url=https://www.bbc.co.uk/news/10364160
}}
* {{cite book
 |last1=Merrifield |first1=Michael
 |last2=Copeland |first2=Ed |author-link2=Edmund Copeland
 |last3=Bowley |first3=Roger
 |year=2010
 |chapter=Neutrinos
 |title=Sixty Symbols
 |publisher=[[University of Nottingham]]
 |chapter-url=http://www.sixtysymbols.com/videos/neutrinos.htm
 |via=[[Brady Haran]]
}}
* {{cite AV media
 |last=Cowan |first=Clyde L. |author-link=Clyde Cowan 
 |title=The Neutrino with Dr.&nbsp;Clyde L. Cowan
 |series=Lecture on Project Poltergeist
 |website=[[YouTube]] |medium=lecture video
 |url=https://www.youtube.com/watch?v=AYqEtm0X2Sc
 |archive-url=https://ghostarchive.org/varchive/youtube/20211030/AYqEtm0X2Sc
 |archive-date=2021-10-30
}}{{cbignore}}
* {{cite web
 |first=Wolfgang
 |last=Pauli
 |author-link=Wolfgang Pauli
 |date=December 1930
 |title=Liebe Radioaktive Damen und Herren
 |work=Bibnum Education
 |trans-title=Dear Radioactive Ladies and Gentlemen
 |translator=Moran, John
 |url=https://www.bibnum.education.fr/physique/physique-nucleaire/chers-mesdames-et-messieurs-radioactifs
 |access-date=25 January 2016
 |archive-date=15 March 2016
 |archive-url=https://web.archive.org/web/20160315140410/https://www.bibnum.education.fr/physique/physique-nucleaire/chers-mesdames-et-messieurs-radioactifs
}} (Pauli's letter stating the hypothesis of the neutrino: online and analyzed; for English version translated by John Moran, click 'The Neutrinos saga').

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[[Category:Dark matter]]
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[[Category:Neutrinos| ]]
[[Category:1930 in science]]