The QCD crossover at zero and non-zero baryon densities from Lattice QCD

TL;DR

This paper maps the QCD crossover line at zero and non-zero baryon densities using lattice QCD, providing detailed temperature and chemical potential dependencies, and analyzing fluctuations to understand the nature of the transition.

Contribution

It presents the first detailed mapping of the QCD crossover line up to fourth order in baryon chemical potential using lattice QCD, including temperature, chemical potential dependencies, and fluctuation analysis.

Findings

Crossover temperature at zero chemical potential is 156.5 MeV.

Curvature of the crossover line is approximately 0.012.

No signs of narrowing of the crossover region for μ_B < 250 MeV.

Abstract

We map out the QCD crossover line $\frac{T_c(\mu_B)}{T_c(0)} = 1 - \kappa_2 \left( \frac{\mu_B}{T_c(0)} \right)^2 - \kappa_4 \left( \frac{\mu_B}{T_c(0)} \right)^4 + \mathcal{O}(\mu_B^6)$ for the first time up to $\mathcal{O}(\mu_B^4)$ for a strangeness neutral system by performing a Taylor expansion of chiral observables in temperature $T$ and chemical potentials $\mu$. At vanishing chemical potential, we report a crossover temperature $T_c(0) = (156.5 \pm 1.5)\;\mathrm{MeV}$ defined by the average of several second-order chiral susceptibilities. For a system with thermal conditions appropriate for a heavy-ion collision, we determined a curvature from the subtracted condensate as $\kappa_2 = 0.0120(20)$ and from the disconnected susceptibility as $\kappa_2 = 0.0123(30)$. The next order $\kappa_4$ is significantly smaller. We also report the crossover temperature as a function of the chemical potentials for: baryon-number, electric charge, strangeness and isospin. Additionally, we find that $T_c(\mu_B)$ is in agreement with lines of constant energy density and constant entropy density. Along this crossover line, we study net baryon-number fluctuations and show that their increase is substantially smaller compared to that obtained in HRG model calculations. Similarly, we analyze chiral susceptibility fluctuations along the crossover line and show that these are constant. We conclude that no signs for a narrowing of the crossover region can be found for baryon chemical potential $\mu_B < 250\;\mathrm{MeV}$.