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Pionium is an exotic atom consisting of one
and one
meson. It can be created, for instance, by interaction of a proton beam accelerated by a particle accelerator and a target nucleus. Pionium has a short lifetime, predicted by chiral perturbation theory to be 2.89×10−15 s. It decays mainly into two
mesons, and to a smaller extent into two photons.

Pionium is currently under investigation at CERN to measure its lifetime. The DIRAC collaboration was able to detect 21227 atomic pairs from a total of 1.5×109 events, which allows the pionium lifetime to be determined to within statistical errors of 9%.[1]

In 2006, the NA48/2 collaboration at CERN published an evidence for pionium production and decay in decays of charged kaons, studying mass spectra of daughter pion pairs in the events with three pions in the final state K± → π±(ππ)atom → π±π0π0.[2] This was followed by a precision measurement of the S-wave pion scattering length, published by the collaboration in 2009.[3]

The results of the above experiments will provide crucial tests of low-energy QCD predictions.[4]

See also[edit]


  1. ^ Adeva, B.; et al. (2011). "Determination of ππ scattering lengths from measurement of π+ππ+π atom lifetime". Physics Letters B. 704 (1-2): 24–29. arXiv:1109.0569Freely accessible. Bibcode:2011PhLB..704...24A. doi:10.1016/j.physletb.2011.08.074. 
  2. ^ Batley, J. R.; et al. (2006). "Observation of a cusp-like structure in the π0π0 invariant mass distribution from K± → π±π0π0 decay and determination of the ππ scattering lengths" (PDF). Physics Letters B. 633 (2–3): 173–182. arXiv:hep-ex/0511056Freely accessible. Bibcode:2006PhLB..633..173N. doi:10.1016/j.physletb.2005.11.087. 
  3. ^ Batley, J. R.; et al. (2009). "Determination of the S-wave ππ scattering lengths from a study of K±π±π0π0 decays" (PDF). European Physical Journal C. 64 (4): 589–608. arXiv:0912.2165Freely accessible. Bibcode:2009EPJC...64..589B. doi:10.1140/epjc/s10052-009-1171-3. 
  4. ^ Santamarina, C.; Schumann, M.; Afanasyev, L. G.; Heim, T. (2003). "A Monte Carlo calculation of the pionium break-up probability with different sets of pionium target cross sections". Journal of Physics B. 36 (21): 4273. arXiv:physics/0306161Freely accessible. Bibcode:2003JPhB...36.4273S. doi:10.1088/0953-4075/36/21/007.