Density screening effects in the NJL model: Chiral condensate, speed of sound, and the Critical End Point

TL;DR

This paper investigates how medium screening effects in the NJL model influence the QCD phase diagram, particularly the chiral transition, CEP location, and related properties, with implications for experiments and compact star physics.

Contribution

It introduces a medium-dependent effective coupling in the NJL model and analyzes its impact on the QCD phase diagram, including the CEP and chiral transition characteristics.

Findings

Screening effects shift the possible position of the Critical End Point.

Screening modifies the nature of the chiral transition.

The speed of sound and chiral condensate behavior are affected by screening.

Abstract

The phase diagram of Quantum Chromodynamics (QCD) remains a central topic in high-energy physics. At high temperature and low baryochemical potential, the chiral transition is experimentally observed and theoretically explored to be a smooth crossover, while at high densities, a first-order phase transition is theoretically expected in lack of direct experimental evidence. The search for the Critical End Point (CEP), where both regimes meet, is one of the main objectives of heavy-ion experiments at FAIR and NICA. In this work, we explore the QCD phase diagram structure using the Nambu--Jona-Lasinio (NJL) model, incorporating medium screening effects through an effective coupling $G(T,\mu)$ for $\mu\gg T\sim 0$. We apply a consistent regularization scheme and Sommerfeld expansion to include low thermal and large density corrections in the gap equation. Our numerical analysis focuses on the behavior of the chiral condensate, the dynamical quark mass, and the speed of sound. We find that screening effects shift the posible position of the CEP and modify the nature of the chiral transition. These findings provide theoretical support for ongoing experimental searches and may have implications for the physics of compact stars.