Pionium is an exotic atom consisting of a π+ 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.