Pionium

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Pionium is an exotic atom consisting of a π⁺ and a π⁻ meson. It is generally created by interaction of a proton beam accelerated by a particle accelerator and a target nucleus.

Currently, pionium is under investigation at CERN to measure its lifetime. The DIRAC collaboration was able to detect more than 5000 pionium break ups from a total of 6.4 x 10⁸ events, which will allow the lifetime to be determined to within statistical errors of 15%. The team hopes to reduce this error to just 10% in the future[1].

In 2005, the NA48/2 collaboration at CERN has found a hint for pionium production 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[2].

The lifetime is predicted by chiral perturbation theory to be 2.89 × 10⁻¹⁵ seconds. The result of the experiment is a crucial test of low-energy QCD predictions[3].

Pionium decays mainly into two π⁰ mesons and to a smaller extent into two photons.