The fragmentation of positronium

Due to collisions with helium atoms has been experimentally investigated. The lightest “atom” made of an electron and a positively charged mate is not hydrogen but positronium, a bound electron–positron pair. Ps lives for only about 140 nanoseconds before its constituents annihilate each other, but that can be long enough to do an experiment. In recent years, physicists have been able to generate Ps beams and measure total cross sections for Ps scattering from various targets. Now, a team of physicists at University College London has conducted an experiment in which Ps scatters inelastically off helium atoms and splits apart. The separated electrons and positrons continue to be highly correlated, and the measured cross section is in good agreement with a coupled-state calculation. A longitudinal-energy peak suggests that some of the resulting electrons are lost to the continuum. (S. Armitage et al., Phys. Rev. Lett. 89 , 173402, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.173402 .)