Return of the prodigal Goldstone boson

We propose that the mass of the η^' meson is a particularly sensitive probe of the properties of finite energy density hadronic matter and quark-gluon plasma. We argue that the mass of the η^' excitation in hot and dense matter should be small, and, therefore, that the η^' production cross section should be much increased relative to that for pp collisions. This may have observable consequences in dilepton and diphoton experiments. {}1996 The American Physical Society.