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

TL;DR

This paper presents evidence for a critical point in the phase diagram of hot, dense nuclear matter through two-pion interferometry measurements and finite-size scaling analysis, indicating a second order phase transition near specific temperature and chemical potential values.

Contribution

It provides experimental and theoretical validation for the existence and properties of the critical end point in the nuclear matter phase diagram, including its universality class and dynamic critical behavior.

Findings

Non-monotonic excitation functions suggest proximity to the critical end point.

Finite-Size Scaling analysis indicates a second order phase transition at the CEP.

Critical exponents place the CEP in the 3D Ising universality class.

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.