Energy loss in perturbative QCD

TL;DR

The paper surveys energy loss of high-energy partons traversing QCD matter, contrasting collisional and radiative mechanisms and highlighting coherent LPM-like suppression in QCD. It presents two complementary theoretical frameworks—the multiple-scattering approach and the light-cone path integral method—that yield the same radiative gluon spectrum, and extends the analysis to expanding plasmas and finite jet-cone effects. Key results include a quadratic L-dependence of radiative energy loss in static media, a pronounced enhancement in hot QGP versus cold matter, and a fundamental link between energy loss and transverse momentum broadening via the transport coefficient q̂. The work discusses phenomenological implications for jet quenching in heavy-ion collisions and advocates for future Monte Carlo modeling and experimental validation to confirm these predictions.

Abstract

We review the properties of energetic parton propagation in hot or cold QCD matter, as obtained in recent works. Advances in understanding the energy loss - collisional and radiative - are summarized, with emphasis on the latter: it features very interesting properties which may help to detect the quark-gluon plasma produced in heavy ion collisions. We describe two different theoretical approaches, which lead to the same radiated gluon energy spectrum. The case of a longitudinally expanding QCD plasma is investigated. The energy lost by a jet with given opening angle is calculated in view of making predictions for the suppression (quenching) of hard jet production. Phenomenological implications for the difference between hot and cold matter are discussed. Numerical estimates of the loss suggest that it may be significantly enhanced in hot compared to cold matter.

Figures (11)

• Figure 1: Self-energy diagrams contributing to the collisional energy loss: (a) in HTL-resummed perturbation theory for soft exchanged momentum and (b) in fixed leading order for hard exchanged momentum.
• Figure 2: Collisional energy loss of a charm quark as a function of its momentum. The quark propagates through an out-of-chemical equilibrium plasma with fugacities $\lambda_g = 1, \lambda_q = 0$ (solid curve) Dirks. The dashed curve is the equilibrium result of Braatena, the dotted curve shows the original prediction by Bjorken Bjorkena .
• Figure 3: Contribution to the induced gluon spectrum by interference between the amplitude $f$ and the Born amplitude $f_{Born}$ .
• Figure 4: Energy loss as a function of the medium size $L$.
• Figure 5: Quark gluon amplitude at time $t$.