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Pionium

Pionium is an exotic atom consisting of a &pi+ and a π- meson. It is generally created by the interaction of a proton beam and a target nucleus in a particle accelerator.

Currently, pionium is under investigation at CERN to measure its lifetime. 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.