Momentum Broadening in Weakly Coupled Quark-Gluon Plasma (with a view to finding the quasiparticles within liquid quark-gluon plasma)

TL;DR

This paper calculates the transverse momentum distribution of energetic partons in weakly coupled quark-gluon plasma, comparing it to strongly coupled models, and suggests experimental ways to identify quasiparticles within the plasma.

Contribution

It provides a detailed calculation of P(k_ot) in weakly coupled quark-gluon plasma using HTL resummation and compares it to holographic models, highlighting potential experimental signatures of quasiparticles.

Findings

P(k_ot) shape is similar at modest k_ot in weakly and strongly coupled plasmas.

P(k_ot) is parametrically larger at high k_ot in weakly coupled plasma.

Large-angle deflections can reveal quasiparticles within the quark-gluon plasma.

Abstract

We calculate P(k_\perp), the probability distribution for an energetic parton that propagates for a distance L through a medium without radiating to pick up transverse momentum k_\perp, for a medium consisting of weakly coupled quark-gluon plasma. We use full or HTL self-energies in appropriate regimes, resumming each in order to find the leading large-L behavior. The jet quenching parameter \hat q is the second moment of P(k_\perp), and we compare our results to other determinations of this quantity in the literature, although we emphasize the importance of looking at P(k_\perp) in its entirety. We compare our results for P(k_\perp) in weakly coupled quark-gluon plasma to expectations from holographic calculations that assume a plasma that is strongly coupled at all length scales. We find that the shape of P(k_\perp) at modest k_\perp may not be very different in weakly coupled and strongly coupled plasmas, but we find that P(k_\perp) must be parametrically larger in a weakly coupled plasma than in a strongly coupled plasma at large enough k_\perp. This means that by looking for rare (but not exponentially rare) large-angle deflections of the jet resulting from a parton produced initially back-to-back with a hard photon, experimentalists can find the weakly coupled short-distance quark and gluon quasiparticles within the strongly coupled liquid quark-gluon plasma produced in heavy ion collisions, much as Rutherford found nuclei within atoms or Friedman, Kendall and Taylor found quarks within nucleons.