What is "pionium"

n. 1 (context physics chemistry English) an exotic atom equivalent to a hydrogen atom with the proton replaced by a pion 2 (context physics English) an onium where the bound antiparticle-particle pair is a pion and an antipion.

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 . 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 events, which allows the pionium lifetime to be determined to within statistical errors of 9%.

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) → πππ. This was followed by a precision measurement of the S-wave pion scattering length, published by the collaboration in 2009.

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