Heavy quark chemical equilibration rate as a transport coefficient

TL;DR

This paper proposes a non-perturbative transport coefficient for heavy quark chemical equilibration, estimating its magnitude at relevant temperatures and discussing its implications for heavy ion collision experiments.

Contribution

It introduces a non-perturbative definition of the heavy quark chemical equilibration rate as a transport coefficient, connecting it to Boltzmann equation results at leading order.

Findings

At T ~ 400 MeV, charm quark equilibration rate is very slow (>60 fm/c).

The rate increases rapidly with temperature.

Non-linear effects can enhance the rate if heavy quarks are initially overabundant.

Abstract

Motivated by indications that heavy (charm and bottom) quarks interact strongly at temperatures generated in heavy ion collision experiments, we suggest a non-perturbative definition of a heavy quark chemical equilibration rate as a transport coefficient. Within leading-order perturbation theory (corresponding to 3-loop level), the definition is argued to reduce to an expression obtained from the Boltzmann equation. Around T ~ 400 MeV, an order-of-magnitude estimate for charm yields a rate Gamma^{-1}_{chem} > 60 fm/c which remains too slow to play a practical role in current experiments. However, the rate increases rapidly with T and, due to non-linear effects, also if the initial state contains an overabundance of heavy quarks.