QCD aboveTc: Hadrons, partons, and the 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 ≲ 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 ≳ 3T c .

Melting hadrons, boiling quarks

“…8 suggests hadron dominance below T = 125 MeV, and quark dominance above T = 150 MeV. This is a decisively more narrow range compared to the wider one seen in the fit in which a mixed parton-hadron phase was used to describe lattice results [52]; see discussion in subsect. 2.5.…”

“…In figs. 10 and 11 in [52] the appearance of partons for T > 140 MeV is noted. Moreover, this model is able to describe precisely the interaction measure…”

“…The dynamical situation is also more complex and one cannot expect that the matter content of the fireball is a parton-hadron ideal mix. The rapid expansion could and should mean that the parton system evolves without having time to enter equilibrium mixing with hadrons, this is normally called super-cooling in the context of a 1st order phase transition, but in context of a mix of partons and hadrons [52], these ideas should also apply: as the parton phase evolves to lower temperature, the yet non-existent hadrons will need to form.…”

“…[52] suggest small if any presence of hadrons. Exploding into space this parton domain dilutes at, or even above, the speed of sound in the transverse direction and even faster into the longitudinal It is likely that this expansion is too fast to allow hadron population to develop from the parton domain.…”

“…However, the physical properties of the mixed system like in the 1st order transition case are obtained in a superposition of fractional gas components. The recent analysis of lattice-QCD results of Biro and Jakovac [52] proceeds in terms of a perfect microscopic mix of partons and hadrons. One should take note that as soon as QCD-partons appear, in such a picture color deconfinement is present.…”