QCD over $T_c$: hadrons, partons and continuum

Abstract

In this paper we provide a physical picture for the QCD phase transition in terms of qualitative changes in the spectral functions. Our approach takes into account the crossover nature of this transition and counts for the observed strong correlation seen in higher order susceptibilities. We demonstrate that the hadron resonance gas, which alone describes the thermodynamics at temperatures $T<T_c$, will appreciably contribute to the total pressure until $T\leq 3T_c$. In this intermediate regime the QCD matter consists of strongly correlated excitations, interpretable as either hadrons or partons. As hadronic spectral peaks gradually vanish, the partonic excitations start to form a stand-alone quasiparticle gas. The conventional picture of a quark gluon plasma emerges only at $T\geq 3T_c$.