  In high energy heavy ion collisions a hot and dense medium is formed, where
the U_A(1) or chiral symmetry may temporarily be restored. As a consequence,
the mass of the eta'(958) mesons may be reduced to its quark model value, and
the abundance of eta' mesons at low p_T may be enhanced by more than a factor
of 10. The intercept parameter lambda_* of the charged pion Bose--Einstein
correlations provides a sensitive observable of the possibly enhanced eta'
abundance. We have analyzed lambda_*(m_T) data from sqrt(s(NN))=200 GeV central
Au+Au reactions measured at the BNL Relativistic Heavy Ion Collider (RHIC),
using extensive Monte Carlo simulations based on six popular models for
hadronic multiplicities. Based on the combined STAR and PHENIX data set, and on
various systematic investigations of resonance multiplicities and model
parameters, we conclude that in sqrt(s(NN))=200 GeV central Au+Au reactions the
mass of the eta' meson is reduced by Delta m > 200 MeV, at the 99.9% confidence
level in the considered model class. Such a significant eta' mass modification
may indicate the restoration of the U_A(1) symmetry in a hot and dense hadronic
matter and the return of the ninth "prodigal" Goldstone boson. A similar
analysis of NA44 S+Pb data at top CERN Super Proton Synchrotron (SPS) energies
showed no significant in-medium eta' mass modification.
