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

Understand

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.