Jet quenching in out-of-equilibrium QCD matter

TL;DR

This paper investigates how the early, out-of-equilibrium stages of quark-gluon plasma affect jet substructure modifications, using advanced theoretical models to compare dynamical and static scenarios.

Contribution

It introduces a novel study of jet substructure modifications during the initial out-of-equilibrium phase of heavy-ion collisions using the Improved Opacity Expansion and kinetic theory.

Findings

Bulk expansion significantly influences jet radiation patterns.

Early-stage out-of-equilibrium matter leaves a sizable imprint on jets.

Comparison with static scenarios highlights the importance of dynamical evolution.

Abstract

We present the first study of jet substructure modifications during the bottom-up evolution that describes the early stages of heavy-ion collisions. To this end, we study the bremsstrahlung radiation rate of soft gluons from a hard parton propagating through out-of-equilibrium QCD matter. The gluon spectrum is computed within the Improved Opacity Expansion, which accounts for both multiple soft and single hard momentum exchanges between the hard probe and the medium. The background evolution is obtained from effective kinetic theory simulations that determine the jet quenching parameter, which in turn controls the radiation rate. We compute the radiation rate for initially under- and over-occupied systems, as well as for an expanding system undergoing hydrodynamization, which typically represents the initial stages of heavy-ion collisions. The results for these dynamical backgrounds are compared to static and thermally matched scenarios, allowing to gauge the importance of bulk expansion in the evolution of the jet cascade. Our findings show that the early stages of the bulk matter evolution in heavy-ion collisions leave a sizable imprint on the radiation pattern inside jets. These results establish a basis for incorporating pre-equilibrium dynamics into realistic descriptions of jet quenching and hard-probe evolution.