Indications for a critical point in the phase diagram for hot and dense nuclear matter

Abstract

Two-pion interferometry measurements are studied for a broad range of collision centralities in Au+Au (Root_s = 7.7 - 200 GeV) and Pb+Pb (Root_s = 2.76 TeV) collisions. They indicate non-monotonic excitation functions for the Gaussian emission source radii difference [(R_out)^2 - (R_side)^2], suggestive of reaction trajectories which spend a fair amount of time near a "soft point" in the equation of state (EOS) that coincides with the critical end point (CEP). A Finite-Size Scaling (FSS) analysis of these excitation functions, provides further validation tests for the CEP. It also indicates a second order phase transition at the CEP, and the values T^{cep} ~ 165 MeV and mu_B^{cep} ~ 95 MeV for its location in the (T, mu_B)-plane of the phase diagram. The static critical exponents (nu ~ 0.66 and gamma ~ 1.2) extracted via the same FSS analysis, place this CEP in the 3D Ising model universality class. A Dynamic Finite-Size Scaling analysis of the excitation functions, gives the estimate z ~ 0.87 for the dynamic critical exponent, suggesting that the associated critical expansion dynamics is dominated by the hydrodynamic sound mode.