Heavy quarks in the early stage of high energy nuclear collisions at RHIC and LHC: Brownian motion versus diffusion in the evolving Glasma

TL;DR

This paper compares charm and beauty quark momentum broadening in the early stages of high energy nuclear collisions, contrasting the effects of evolving Glasma fields with standard Brownian motion in a thermal medium, revealing the influence of gluon field coherence.

Contribution

It introduces a comparison between diffusion in evolving Glasma fields and Markovian Brownian motion, highlighting the role of gluon field coherence in heavy quark momentum broadening.

Findings

At low saturation scale, $Q_s$, diffusion in Glasma matches perturbative Brownian motion.

At high $Q_s$, Glasma causes faster momentum broadening due to strong gluon field coherence.

Diffusion coefficients in Glasma are consistent with perturbative calculations at low $Q_s$.

Abstract

We study the transverse momentum, $p_T$, broadening of charm and beauty quarks in the early stage of high energy nuclear collisions. We aim to compare the diffusion in the evolving Glasma fields with that of a standard, Markovian-Brownian motion in a thermalized medium with the same energy density of the Glasma fields. The Brownian motion is studied via Langevin equations with diffusion coefficients computed within perturbative Quantum Chromodynamics. We find that for small values of the saturation scale, $Q_s$, the average $p_T$ broadening in the two motions is comparable, that suggests that the diffusion coefficient in the evolving Glasma fields is in agreement with the perturbative calculations. On the other hand, for large $Q_s$ the $p_T$ broadening in the Langevin motion is smaller than that in the Glasma fields. This difference can be related to the fact that heavy quarks in the latter system experience diffusion in strong, coherent gluon fields that leads to a faster momentum broadening due to memory, or equivalently to a strong correlation in the transverse plane.