Crossover-model approach to QCD phase diagram, equation of state and susceptibilities in the 2 + 1 and 2 + 1 + 1 flavor systems

We construct a simple model for describing the hadron–quark crossover transition by using lattice QCD (LQCD) data in the 2 + 1 flavor system, and draw the phase diagram in the 2 + 1 and 2 + 1 + 1 flavor systems through analyses of the equation of state (EoS) and the susceptibilities. In the present hadron–quark crossover (HQC) model, the entropy density s is defined by s = fHsH + (1 − fH)sQ with the hadron-production probability fH, where sH is calculated by the hadron resonance gas model that is valid in low temperature (T) and sQ is evaluated by the independent quark model that explains LQCD data on the EoS in the region 400 ≲ T ≤ 500MeV for the 2 + 1 flavor system and 400 ≲ T ≤ 1000MeV for the 2 + 1 + 1 flavor system. The fH is determined from LQCD data on s and susceptibilities for the baryon-number (B), the isospin (I) and the hypercharge (Y ) in the 2 + 1 flavor system. The HQC model is successful in reproducing LQCD data on the EoS and the flavor susceptibilities χff′(2) for f, f′ = u, d, s in the ...