Spin relaxation rate for heavy quarks in weakly coupled QCD plasma

TL;DR

This paper calculates the spin relaxation rate for heavy quarks in a weakly coupled QCD plasma using three different methods, revealing it is smaller than other non-hydrodynamic modes and connecting these approaches through Ward-Takahashi identities.

Contribution

It provides the first detailed calculation of the heavy-quark spin relaxation rate in perturbative QCD and compares three distinct evaluation methods.

Findings

Spin relaxation rate scales as g^4 log(1/g) T (T/M)^2.

The rate is smaller than other non-hydrodynamic modes.

Three methods for evaluation are demonstrated and connected.

Abstract

We compute the relaxation rate of the spin density of heavy quarks in a perturbative QCD plasma to leading-log order in the coupling constant $g$. The spin relaxation rate $\Gamma_s$ in spin hydrodynamics is shown to be $\Gamma_s\sim g^4\log(1/g)T (T/M)^2$ in the heavy-quark limit $T/M\ll 1$, which is smaller than the relaxation rate of other non-hydrodynamic modes by additional powers of $T/M$. We demonstrate three different methods to evaluate the spin relaxation rate: 1) the Green-Kubo formula in the spin hydrodynamic regime, 2) the spin density correlation function in the strict hydrodynamic limit, and 3) quantum kinetic theory of the spin distribution function in momentum space. We highlight the interesting differences between these methods, while they are ultimately connected to each other by the underlying Ward-Takahashi identity for the non-conserved spin density.