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Significant in-medium eta' mass reduction in sqrt(s(NN))=200 GeV Au+Au collisions at the BNL Relativistic Heavy Ion Collider

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.