Strangeness-Correlations on the pseudo-critical line in (2+1)-flavor QCD

Abstract

We present some lattice QCD results on first ($\chi_1^i$) and second ($\chi_2^i$) cumulants of and correlations ($\chi_{11}^{ij}$) among net baryon-number ($B$), strangeness ($S$) and electric charge ($Q$) along the pseudo-critical line ($T_{pc}(\mu_B)$) in the temperature ($T$)--baryon chemical potential ($\mu_B$) phase diagram of (2+1)-flavor QCD. We point out that violations of the isospin symmetric limit of vanishing electric charge chemical potential are small along the $T_{pc}(\mu_B)$ for the entire range of $\mu_B$ covered in the RHIC beam energy scan. For the strangeness neutral matter produced in heavy-ion collisions this leads to a close relation between $\chi_{11}^{BS}$ and $\chi_{11}^{QS}$. We compare lattice QCD results for $\chi_{11}^{BS}/\chi_2^S$ along the $T_{pc}(\mu_B)$ line with preliminary experimental measurements of $\chi_{11}^{BS}/\chi_2^S$ for collision energies $7.7~{\rm GeV}\le \sqrt{s_{_{NN}}}\le 62.4~{\rm GeV}$. While we find good agreements for $\sqrt{s_{_{NN}}}\ge 39$~GeV, differences are sizeable at smaller values of $\sqrt{s_{_{NN}}}$. Moreover, we compare lattice QCD results for the ratio of the strangeness ($\mu_S$) to baryon ($\mu_B$) chemical potentials, which define a strangeness neutral system with fixed electric charge to baryon number density, with experimental results obtained by the STAR collaboration for $\mu_S/\mu_B$ using strange baryon yields on the freeze-out line. Finally, we determine the baryon chemical potential at the freeze-out ($\mu_B^f$) by comparing $\chi_1^B/\chi_2^B$ along the $T_{pc}(\mu_B)$ with the experimentally measured net-proton cumulants $\chi_1^p/\chi_2^p$. We find that $\{\mu_B^f, T_{pc}(\mu_B^f) \}$ are consistent with the freeze-out parameters of the statistical-model fits to experimentally measured hadron yields for $\sqrt{s_{_{NN}}} \geq 11.5$ GeV.