1) Exotic atom 
Pionium is an exotic atom consisting of one {SubatomicParticle|link=yes|Pion+} and one {SubatomicParticle|link=yes|Pion-} 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 {val|2.89|e=-15|u=s...