Pionium
Pionium is an exotic atom consisting of one π+ and one π− 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 2.89×10 s. It decays mainly into two π0 mesons, and to a smaller extent into two photons.