View source for Pionium

{{Short description|Composite particle of two mesons}}
{{Antimatter}}
'''Pionium''' is a [[composite particle]] consisting of one {{SubatomicParticle|link=yes|Pion+}} and one {{SubatomicParticle|link=yes|Pion-}} [[meson]]. It can be created, for instance, by interaction of a [[proton]] beam accelerated by a [[particle accelerator]] and a target [[atomic nucleus|nucleus]]. Pionium has a short lifetime, predicted by [[Chirality (physics)|chiral]] [[perturbation theory (quantum mechanics)|perturbation theory]] to be {{val|2.89|e=-15|u=s}} (i.e. 2.89 [[Femtosecond|femtoseconds]]). It decays mainly into two {{SubatomicParticle|link=yes|Pion0}} [[mesons]], and to a smaller extent into two photons.

It has been investigated at [[CERN]] to measure its lifetime. The Dimeson Relativistic Atomic Complex (DIRAC) experiment at the [[Proton Synchrotron]] was able to detect 21,227 atomic pairs from a total of {{val|1.5|e=9}} events, which allows the pionium lifetime to be determined to within statistical errors of 9%.<ref name="hep-ph/9808407">
{{cite journal
 |last1=Adeva |first1=B.
 |display-authors=etal
 |year=2011
 |title=Determination of ''ππ'' scattering lengths from measurement of ''π''<sup>+</sup>''π''<sup>−</sup>''π''<sup>+</sup>''π''<sup>−</sup> atom lifetime
 |journal=[[Physics Letters B]]
 |volume=704 |issue=1–2 |pages=24–29
 |arxiv=1109.0569
 |doi=10.1016/j.physletb.2011.08.074
 |bibcode=2011PhLB..704...24A
|s2cid=18091499
 }}</ref>

In 2006, the [[NA48 experiment|NA48/2 collaboration]] at CERN published an evidence for pionium production and decay in decays of charged [[kaon]]s, studying mass spectra of daughter [[pion]] pairs in the events with three pions in the final state K<sup>±</sup>&nbsp;→&nbsp;π<sup>±</sup>(ππ)<sub>atom</sub>&nbsp;→&nbsp;π<sup>±</sup>π<sup>0</sup>π<sup>0</sup>.<ref name="hep-ex/0511056">
{{cite journal
 |last1=Batley |first1=J. R.
 |display-authors=etal
 |year=2006
 |title=Observation of a cusp-like structure in the ''π''<sup>0</sup>''π''<sup>0</sup> invariant mass distribution from ''K''<sup>±</sup>&nbsp;→&nbsp;''π''<sup>±</sup>''π''<sup>0</sup>''π''<sup>0</sup> decay and determination of the ''ππ'' scattering lengths
 |journal=[[Physics Letters B]]
 |volume=633 |issue=2–3 |pages=173–182
 |arxiv=hep-ex/0511056
 |bibcode=2006PhLB..633..173N
 |doi=10.1016/j.physletb.2005.11.087
|s2cid=14747304
 }}</ref> This was followed by a precision measurement of the S-wave pion [[scattering length]], published by the collaboration in 2009.<ref name=na48>
{{cite journal
 |last1=Batley |first1=J. R.
 |display-authors=etal
 |year=2009
 |title=Determination of the S-wave ''ππ'' scattering lengths from a study of ''K''<sup>±</sup>→''π''<sup>±</sup>''π''<sup>0</sup>''π''<sup>0</sup> decays
 |journal=[[European Physical Journal C]]
 |volume=64 |issue=4 |pages=589–608
 |arxiv=0912.2165
 |bibcode=2009EPJC...64..589B
 |doi=10.1140/epjc/s10052-009-1171-3
|s2cid=14006827
 }}</ref>

The results of the above experiments will provide crucial tests of low-energy [[Quantum chromodynamics|QCD]] predictions.<ref name="urlA Monte Carlo calculation of the pionium break-up probability">
{{cite journal
 |last1=Santamarina |first1=C.
 |last2=Schumann |first2=M.
 |last3=Afanasyev |first3=L. G.
 |last4=Heim |first4=T.
 |year=2003
 |title=A Monte Carlo calculation of the pionium break-up probability with different sets of pionium target cross sections
 |journal=[[Journal of Physics B]]
 |volume=36 |issue=21 |pages=4273–4287
 |arxiv=physics/0306161
 |bibcode=2003JPhB...36.4273S
 |doi=10.1088/0953-4075/36/21/007
|citeseerx=10.1.1.286.4685
 |s2cid=250810773
 }}</ref>

==See also==
*[[Mesonic molecule]]
*[[Onium]]

==References==
{{reflist}}

{{Particles}}

[[Category:Mesons]]
[[Category:Onia]]