Pionium

Pionium is an exotic atom consisting of one π+
 and one π−
 mesons. 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 more than 5000 pionium decays from a total of 6.4×108 events, which will allow the lifetime to be determined to within statistical errors of 15%. The team hopes to reduce this error to 10% in the future.[1]

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: K^\pm\to\pi^\pm(\pi\pi)_{atom}\to\pi^\pm\pi^0\pi^0.[2] The possibility to measure pionium characteristics is being investigated.[3]

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

See also

References

  1. ^ Schacher, J. (1998). "DIRAC Experiment at CERN: Lifetime Measurement of Pionium". arXiv:hep-ph/9808407 [hep-ph]. 
  2. ^ 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. 
  3. ^ Madigozhin, D. (2005). "Pion scattering lengths from NA48". arXiv:hep-ex/0510004 [hep-ex]. 
  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/0306161. Bibcode 2003JPhB...36.4273S. doi:10.1088/0953-4075/36/21/007.