Significant in-medium<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mi>η</mml:mi><mml:mrow><mml:mo>′</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>mass reduction in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msqrt><mml:mrow><mml:msub><mml:mi>s</mml:mi><mml:mrow><mml:mi mathvariant="italic">NN</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:msqrt></mml:mrow></mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mo>=</mml:mo></mml:mrow></mml:math> 200 GeV Au<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:math>Au collisions at the BNL Relativistic Heavy Ion Collider

In high-energy heavy ion collisions a hot and dense medium is formed, where the ${\mathrm{U}}_{\mathrm{A}}(1)$ or chiral symmetry may temporarily be restored. As a consequence, the mass of the ${\ensuremath{\eta}}^{\ensuremath{'}}$(958) mesons may be reduced to its quark model value, and the abundance of ${\ensuremath{\eta}}^{\ensuremath{'}}$ mesons at low ${p}_{\mathrm{T}}$ may be enhanced by more than a factor of 10. The intercept parameter ${\ensuremath{\lambda}}_{*}$ of the charged pion Bose Einstein correlations provides a sensitive observable of the possibly enhanced ${\ensuremath{\eta}}^{\ensuremath{'}}$ abundance. We have analyzed ${\ensuremath{\lambda}}_{*}({m}_{\mathrm{T}})$ data from $\sqrt{{s}_{{}_{\mathit{NN}}}}=200 \mathrm{GeV}$ 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 $\sqrt{{s}_{{}_{\mathit{NN}}}}=200 \mathrm{GeV}$ central Au$+$Au reactions the mass of the ${\ensuremath{\eta}}^{\ensuremath{'}}$ meson is reduced by $\ensuremath{\Delta}{m}_{{\ensuremath{\eta}}^{\ensuremath{'}}}^{*}&gt;200 \mathrm{MeV}$, at the 99.9% confidence level in the considered model class. Such a significant ${\ensuremath{\eta}}^{\ensuremath{'}}$ 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 Synchroton (SPS) energies showed no significant in-medium ${\ensuremath{\eta}}^{\ensuremath{'}}$ mass modification.