Neutron star mass limit at $2M_\odot$ supports the existence of a CEP

TL;DR

The paper argues that the existence of neutron stars with masses around 2 solar masses supports the presence of a critical endpoint in the QCD phase diagram, indicating a strong first-order phase transition in dense matter.

Contribution

It links neutron star observations to the existence of a critical endpoint in the QCD phase diagram through the analysis of mass-radius characteristics and phase transition signatures.

Findings

Presence of a 'horizontal branch' suggests a strong first-order phase transition.

Correlation between the maximum radius of the horizontal branch and transition pressure.

Implications for upcoming heavy-ion collision experiments at NICA and FAIR.

Abstract

We point out that the very existence of a "horizontal branch" in the mass-radius characteristics for neutron stars indicates a strong first-order phase transition and thus supports the existence of a critical endpoint (CEP) of first order phase transitions in the QCD phase diagram. This branch would sample a sequence of hybrid stars with quark matter core, leading to the endpoint of stable compact star configurations with the highest possible baryon densities. Since we know of the existence of compact stars with $2M_\odot$, this hypothetical branch has to lie in the vicinity of this mass value, if it exists. We report here a correlation between the maximal radius of the horizontal branch and the pressure at the onset of hadron-to-quark matter phase transition which is likely to be a universal quantity of utmost relevance to the upcoming experiments with heavy-ion collisions at NICA and FAIR.