Effective Partonic Entropy Suppression Near the QCD Crossover

The QCD crossover at vanishing baryochemical potential and high temperature involves a smooth change from hadronic to partonic degrees of freedom. A phenomenological equation of state should therefore describe not only the activation of quark–gluon degrees of freedom, but also the fact that the plasma remains strongly interacting near the crossover.

In the present work, we consider an entropy-based crossover construction in which the total entropy density is written as a smooth combination of a hadronic baseline and an effective QGP contribution. The hadronic and partonic sectors are controlled by complementary crossover weights, while the non-ideal character of the partonic phase is encoded through a temperature-dependent entropy-suppression factor Rs(T). Once the entropy density is specified, the pressure, energy density, interaction measure, specific heat, and squared speed of sound are obtained from thermodynamic identities rather than fitted independently.

The suppression factor separates two physical effects: the gradual activation of partonic degrees of freedom and their delayed approach to an ideal QGP entropy. This construction gives a smooth entropy profile and allows derivative-sensitive observables, such as the speed of sound, to remain thermodynamically consistent. A representative comparison with lattice-QCD thermodynamic trends shows that the main crossover behavior can be captured by the effective release and suppression of degrees of freedom.

We conclude that entropy-level modeling provides a compact and physically transparent way to describe the QCD crossover. In addition, the effective suppression factor Rs(T) offers a simple representation of the non-ideal nature of the QGP near the transition.