Momentum broadening in unstable quark-gluon plasma

TL;DR

This paper investigates how unstable, anisotropic quark-gluon plasma affects the momentum broadening of high-energy partons, revealing exponential growth of the broadening parameter over time and its directional dependence, impacting jet quenching phenomenology.

Contribution

It develops a formalism to compute the momentum broadening parameter in unstable plasmas, focusing on extremely oblate configurations relevant to heavy ion collisions, and uncovers its exponential time dependence.

Findings

$$ grows exponentially with time in unstable plasma.

Momentum broadening is highly direction-dependent.

Short-time $$ is comparable to equilibrium value.

Abstract

Quark-gluon plasma produced at the early stage of ultrarelativistic heavy ion collisions is unstable, if weakly coupled, due to the anisotropy of its momentum distribution. Chromomagnetic fields are spontaneously generated and can reach magnitudes much exceeding typical values of the fields in equilibrated plasma. We consider a high energy test parton traversing an unstable plasma that is populated with strong fields. We study the momentum broadening parameter $\hat q$ which determines the radiative energy loss of the test parton. We develop a formalism which gives $\hat q$ as the solution of an initial value problem, and we focus on extremely oblate plasmas which are physically relevant for relativistic heavy ion collisions. The parameter $\hat q$ is found to be strongly dependent on time. For short times it is of the order of the equilibrium value, but at later times $\hat q$ grows exponentially due to the interaction of the test parton with unstable modes and becomes much bigger than the value in equilibrium. The momentum broadening is also strongly directionally dependent and is largest when the test parton velocity is transverse to the beam axis. Consequences of our findings for the phenomenology of jet quenching in relativistic heavy ion collisions are briefly discussed.