Observation of the critical end point in the phase diagram for hot and dense nuclear matter

Abstract

Excitation functions for the Gaussian emission source radii difference ($R^2_{\text{out}} - R^2_{\text{side}}$) obtained from two-pion interferometry measurements in Au+Au ($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV) collisions, are studied for a broad range of collision centralities. The observed non-monotonic excitation functions validate the finite-size scaling patterns expected for the deconfinement phase transition and the critical end point (CEP), in the temperature vs. baryon chemical potential ($T,\mu_B$) plane of the nuclear matter phase diagram. A Finite-Size Scaling (FSS) analysis of these data indicate a second order phase transition with the estimates $T^{\text{cep}} \sim 165$~MeV and $\mu_B^{\text{cep}} \sim 95$~MeV for the location of the critical end point. The critical exponents ($\nu \sim 0.66$ and $\gamma \sim 1.2$) extracted via the same FSS analysis, places the CEP in the 3D Ising model universality class.