Charm Fluctuations and Deconfinement

Abstract

We establish that the charmed hadrons start dissociating at the chiral crossover temperature, ${T_{pc}}$, leading to the appearance of charm degrees freedom carrying fractional baryon number. Our method is based on analyzing the second and fourth-order cumulants of charm (${C}$) fluctuations, and their correlations with baryon number (${B}$), electric charge (${Q}$) and strangeness (${S}$) fluctuations. The first-time calculation of the ${QC}$ correlations on the high statistics datasets of the HotQCD Collaboration enables us to disentangle the contributions from different electrically-charged charm subsectors in the hadronic phase. In particular, we see an enhancement over the PDG expectation in the fractional contribution of the ${|Q|}=2$ charm subsector to the total charm partial pressure for ${T<T_{pc}}$; this enhancement is in agreement with the Quark Model extended Hadron Resonance Gas (QM-HRG) model calculations. Furthermore, the agreement of QM-HRG calculations with the projections onto charmed baryonic and mesonic correlations in different charm subsectors indicates the existence of not-yet-discovered charmed hadrons in all charm subsectors below ${T_{pc}}$. We aim at determining the relevant degrees of freedom in temperature range ${T_{pc}<T<340 \text{ MeV}}$ by assuming the existence of a non-interacting gas of charmed quasi-particles composed of meson, baryon and quark-like excitations above $T_{pc}$. Our data suggest that the particles with quantum numbers consistent with quarks start appearing at $T_{pc}$.