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.