  We describe an experimental test of whether particle decay causes wave
function collapse. The test uses interference between two well separated, but
coherent, sources of vector mesons. The short-lived mesons decay before their
wave functions can overlap, so any interference must involve identical final
states. Unlike previous tests of nonlocality, the interference involves
continuous variables, momentum and position. Interference can only occur if the
wave function retains amplitudes for all possible decays. The interference can
be studied through the transverse momentum spectrum of the reconstructed
mesons.
