A New State of Matter between the Hadronic Phase and the Quark-Gluon Plasma?

N3AS-25-009

A New State of Matter between the Hadronic Phase and the Quark-Gluon Plasma?

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Abstract

Lattice-QCD simulations and theoretical arguments hint at the existence of an intermediate phase of strongly interacting matter between a confined hadron gas and a deconfined Quark-Gluon Plasma (QGP). We qualitatively and semi-quantitatively explore and differentiate the phase structures in the temperature window from the QCD pseudo-critical temperature T_c\simeq 160\;\text{MeV} to the pure-gluonic deconfinement temperature T_d\simeq 285\;\text{MeV}. We propose a three-regime picture using a hadron resonance gas (HRG) description augmented with the glueball spectrum based on the analysis of a large number, N_c, of colors. We estimate the entropy density from our model to confirm that the lattice-QCD data are bracketed with three regimes, i.e., a hadron gas, a QGP, and a new phase for T_c \lesssim T \lesssim T_d. In this new phase that we name a Spaghetti of Quarks with Glueballs (SQGB), thermal degrees of freedom of quarks are deconfined, yet gluons remain confined in glueballs. Since the Hagedorn temperature, T_H\sim 285\;\text{MeV}, is universal in the meson and the glueball sectors, in the infinite N_c limit, the phase diagram in the plane of the baryon chemical potential and the temperature is reduced to the one with the confined and deconfined phases and Quarkyonic Matter at high density. At large but finite N_c, an SQGB window may open between these phases. We point out that the SQGB has interesting similarities with Quarkyonic Matter and that this matter in the large N_c limit is confined as measured by the interaction between heavy quarks, but behaves in other respects like a quasi-free gas of quarks. As a result of the extrapolation to N_c=3, we present a revised phase diagram with the SQGB phase bounded by thermal crossovers. Finally, we give a quantitative analysis of chiral symmetry restoration in the SQGB phase.

Associated Fellows