Are there hadronic bound states above the QCD transition temperature?

TL;DR

This study uses lattice QCD to explore the existence of hadronic bound states in the quark-gluon plasma phase above the critical temperature, suggesting that such states may persist in the deconfined regime.

Contribution

First lattice QCD investigation into the formation of bound states in the equilibrated, parton-dominated phase above the transition temperature.

Findings

Non-diagonal quark correlators are finite up to about 2 T_c.

Evidence suggests coexistence of quasiparticles and bound states above T_c.

Color-neutral bound states may exist in the deconfined phase.

Abstract

Recent lattice QCD calculations, at physical pion masses and small lattice spacings that approach the continuum limit, have revealed that non-diagonal quark correlators above the critical temperature are finite up to about 2 $T_c$. Since the transition from hadronic to free partonic degrees of freedom is merely an analytic cross-over, it is likely that, in the temperature regime between 1-2 $T_c$, quark and gluon quasiparticles and pre-hadronic bound states can coexist. The correlator values, in comparison to PNJL model calculations beyond mean-field, indicate that at least part of the mixed phase resides in color-neutral bound states. A similar effect was postulated for the in-medium fragmentation process, i.e. for partons which do not thermalize with the system and thus constitute the non-equilibrium component of the particle emission spectrum from a deconfined plasma phase. Here, for the first time we investigate the likelihood of forming bound states also in the equilibrated, parton dominated phase above $T_c$ which is described by lattice QCD.