Pionium is an exotic atom consisting of one π+ and one π− 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 π0 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%. 
In 2005, 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: . The possibility to measure pionium characteristics is being investigated.
- Adeva, B.; et al. (DIRAC Collaboration) (2011). "Determination of ππ scattering lengths from measurement of π+π−π+π− atom lifetime". Physics Letters B 704 (1-2): 24–29. arXiv:1109.0569. doi:10.1016/j.physletb.2011.08.074.
- Batley, J. R.; et al. (NA48/2 Collaboration) (2005). "Observation of a cusp-like structure in the π0π0 invariant mass distribution from K± → π±π0π0 decay and determination of the ππ scattering lengths". Physics Letters B 633 (2–3): 173–182. arXiv:hep-ex/0511056. Bibcode:2006PhLB..633..173N. doi:10.1016/j.physletb.2005.11.087.
- Madigozhin, D. (2005). "Pion scattering lengths from NA48". arXiv:hep-ex/0510004 [hep-ex].
- 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/0306161. Bibcode:2003JPhB...36.4273S. doi:10.1088/0953-4075/36/21/007.
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