Abstract
A search for the formation of free pionium (π+e-) in vacuum, carried out at the Swiss Institute for Nuclear Research (SIN, now called PSI), has been reported by H J Mundinger and colleagues (Europhys. Lett. 8 339). Pionium is an 'exotic atom' of hydrogen, more usefully identified as an exotic hydrogen isotope, in which the proton has been replaced by a positive pion. Such isotopes – more common examples being positronium (e+e-) and muonium (μ+e-) – are indispensable for studying such fundamental problems areas the relativistic two-body problem in quantum electrodynamics.