Charm mass effects in bulk channel correlations

TL;DR

This paper investigates the influence of charm quarks on the bulk viscosity of high-temperature QCD matter, using perturbative calculations of correlators and comparing with lattice data to understand non-perturbative effects.

Contribution

It provides a next-to-leading order perturbative calculation of the scalar density correlator relevant for charm effects and compares it with lattice data, highlighting discrepancies and non-perturbative insights.

Findings

Discrepancies between NLO perturbation theory and lattice data are larger in the scalar channel.

Non-perturbative effects may be significant in the scalar channel compared to the vector channel.

A transport peak in the spectral function encodes charm quark chemical equilibration information.

Abstract

The bulk viscosity of thermalized QCD matter at temperatures above a few hundred MeV could be significantly influenced by charm quarks because their contribution arises four perturbative orders before purely gluonic effects. In an attempt to clarify the challenges of a lattice study, we determine the relevant imaginary-time correlator (of massive scalar densities) up to NLO in perturbation theory, and compare with existing data. We find discrepancies much larger than in the vector channel; this may hint, apart from the importance of taking a continuum limit, to larger non-perturbative effects in the scalar channel. We also recall how a transport peak related to the scalar density spectral function encodes non-perturbative information concerning the charm quark chemical equilibration rate close to equilibrium.