Phys. Rev. Lett. 102, 213401 (2009)

Muons are heavy, unstable cousins of electrons that are produced by high-energy collisions of more commonplace particles.

Since the 1960s, high-energy physicists have observed atomic-like 'muonium' particles made up of an antimuon and an electron, or 'muonic atoms' in which a muon replaces an ordinary atomic electron. But nobody has seen 'true muonium', made up of a muon and an antimuon. Now Stanley Brodsky of the SLAC National Accelerator Laboratory at Stanford University in California and Richard Lebed of Arizona State University in Tempe suggest that physicists could spot the stuff if they just look in the right place.

They show that true muonium can briefly appear in colliding beams of electrons and their antiparticles, positrons. In fact, the authors say, true muonium may have already been made in modern electron-positron colliders.