Strangeness-Correlations on the pseudo-critical line in (2+1)-flavor QCD
We present some lattice QCD results on first ($χ_1^i$) and second ($χ_2^i$) cumulants of and correlations ($χ_{11}^{ij}$) among net baryon-number ($B$), strangeness ($S$) and electric charge ($Q$) along the pseudo-critical line ($T_{pc}(μ_B)$) in the temperature ($T$)--baryon chemical potential ($μ_B$) 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 $T_{pc}(μ_B)$ for the entire range of $μ_B$ covered in the RHIC beam energy scan. For the strangeness neutral matter produced in heavy-ion collisions this leads to a close relation between $χ_{11}^{BS}$ and $χ_{11}^{QS}$. We compare lattice QCD results for $χ_{11}^{BS}/χ_2^S$ along the $T_{pc}(μ_B)$ line with preliminary experimental measurements of $χ_{11}^{BS}/χ_2^S$ 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 ($μ_S$) to baryon ($μ_B$) 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 $μ_S/μ_B$ using strange baryon yields on the freeze-out line. Finally, we determine the baryon chemical potential at the freeze-out ($μ_B^f$) by comparing $χ_1^B/χ_2^B$ along the $T_{pc}(μ_B)$ with the experimentally measured net-proton cumulants $χ_1^p/χ_2^p$. We find that $\{μ_B^f, T_{pc}(μ_B^f) \}$ 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.