The baryon number fluctuation $\kappa\sigma^2$ as a probe of nuclear matter phase transition at high baryon density

TL;DR

This paper proposes using the kurtosis of baryon number fluctuations as a sensitive indicator to identify phase transitions in nuclear matter at high baryon densities, linking theoretical predictions with experimental observations.

Contribution

It introduces the idea that baryon number fluctuation kurtosis can serve as a probe for nuclear matter phase transitions at high baryon density, connecting phase boundary features with experimental data.

Findings

Peak of $oldsymbol{ ext{kappa}\sigma^2}$ around 5 GeV collision energy indicating the chiral phase transition CEP.

Negative $oldsymbol{ ext{kappa}\sigma^2}$ at low energies associated with the nuclear liquid-gas phase transition.

Experimental measurements at HADES and STAR are consistent with the theoretical predictions.

Abstract

Two critical end points (CEPs) of the chiral phase transition and the nuclear liquid-gas phase transition show up at finite baryon chemical potential. The kurtosis $\kappa\sigma^2$ of baryon number fluctuation on the $T-\mu_B$ plane is positive on the first-order side and negative on the crossover side along the phase boundary. The freeze-out line extracted from the heavy ion collisions crosses between these two phase boundaries, one can observe a peak of $\kappa\sigma^2$ around the collision energy $5 {\rm GeV}$ near the CEP of the chiral phase transition, and negative $\kappa\sigma^2$ at low collision energies due to the CEP of the nuclear liquid-gas phase transition. This expalains the experimental measurement of $\kappa\sigma^2$ at the collision energies of 2.4 GeV at HADES and 3 GeV and 7.7-200 GeV at STAR for most central collision. Thus we propose that the baryon number fluctuation $\kappa\sigma^2$ can be used as a probe of nuclear matter phase structure at high baryon density.