Abstract
We present results for pseudo-critical temperatures of QCD chiral crossovers at zero and non-zero values of baryon (B), strangeness (S), electric charge (Q), and isospin (I) chemical potentials μX=B,Q,S,I. The results were obtained using lattice QCD calculations carried out with two degenerate up and down dynamical quarks and a dynamical strange quark, with quark masses corresponding to physical values of pion and kaon masses in the continuum limit. By parameterizing pseudo-critical temperatures as Tc(μX)=Tc(0)[1−κ2X(μX/Tc(0))2−κ4X(μX/Tc(0))4], we determined κ2X and κ4X from Taylor expansions of chiral observables in μX. We obtained a precise result for Tc(0)=(156.5±1.5) MeV. For analogous thermal conditions at the chemical freeze-out of relativistic heavy-ion collisions, i.e., μS(T,μB) and μQ(T,μB) fixed from strangeness-neutrality and isospin-imbalance, we found κ2B=0.012(4) and κ4B=0.000(4). For μB≲300 MeV, the chemical freeze-out takes place in the vicinity of the QCD phase boundary, which coincides with the lines of constant energy density of 0.42(6)GeV/fm3 and constant entropy density of 3.7(5)fm−3.
Highlights
The spontaneous breaking of the chiral symmetry in quantum chromodynamics (QCD) is a key ingredient for explaining the masses of hadrons that constitute almost the entire mass of our visible Universe
Since the singlet-axial UA(1) symmetry of QCD is expected to remain broken at all T, the quark-line connected piece is expected to remain finite even for mu = md → 0
Physical values of mu,d might not reside within the scaling regime of the second order chiral phase transition; chiral observables may contain additional non-singular, polynomial in m, corrections
Summary
The spontaneous breaking of the chiral symmetry in quantum chromodynamics (QCD) is a key ingredient for explaining the masses of hadrons that constitute almost the entire mass of our visible Universe. The chiral crossover in the early Universe took place at vanishingly small baryon chemical potential μB, the electric charge chemical potential μQ at that stage might have been non-vanishing [2]. The phase structure of QCD-matter in the T μB plane can be probed in various ongoing and upcoming relativistic heavy-ion collision experiments [4]. The phase diagram of QCD can be explored in these experiments if the so-called chemical freeze-out takes place in the proximity of the chiral crossover phase boundary in the T -μB plane [5]. With the aid of state-of-the-art lattice-regularized QCD calculations this work aims at determining chiral pseudocritical temperatures in QCD at zero and non-zero chemical potentials μB,Q,S, as well as for the situation analogous to the chemical freeze-out stage of relativistic heavyion collision experiments.
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