This book, an M.Sc. thesis in experimental nuclear physics, reports on a difficult and unresolved puzzle of relativistic heavy ion collisions: the enhanced production of low-mass lepton pairs in 200 GeV Au+Au collisions, as observed by the PHENIX experiment at Brookhaven National Laboratory's Relativistic Heavy Ion Collider. Two partial explanations are given: The first is a possible in-medium mass modification of the eta' mesons, a signal of a symmetry restoration in hot and dense hadronic matter, inspired by theoretical models and a recent indirect measurement. These in-medium modified eta' mesons leave behind a fingerprint, their enhanced production in vacuum, that lead to enhanced dilepton production via the Dalitz-decay of eta' - (e+ e- photon). As the enhancement is restricted to the soft transverse momentum region of pT