Generalized susceptibilities and the properties of charm degrees of freedom across the QCD crossover temperature

TL;DR

This study investigates charm susceptibilities in lattice QCD, showing HRG models work below the crossover but fail above, where charm quarks become significant, indicating a transition in charm degrees of freedom.

Contribution

It provides continuum estimates of charm partial pressures and demonstrates the breakdown of HRG description above the chiral crossover in QCD.

Findings

HRG model describes charm susceptibilities below crossover

HRG description breaks down just above crossover

Charm quark pressure becomes significant above crossover

Abstract

We study the generalized charm susceptibilities in 2+1 flavor QCD on the lattice at several lattice spacings. We show that, below the chiral crossover, these susceptibilities are well described by the hadron resonance gas (HRG) model if charmed hadrons not listed in tables of the Particle Data Group are included. However, the HRG description abruptly breaks down just above the chiral crossover. To understand this, we use a model for the charm pressure in which it is expressed as the sum of partial pressures from charmed baryons, charmed mesons, and charm quarks. We present continuum estimates of these partial pressures and find that, while the partial pressures of charmed mesons and baryons drop below their respective HRG predictions, the charm quark pressure becomes non-zero above the chiral crossover.