Jet quenching in out-of-equilibrium QCD matter

TL;DR

The paper tackles how jets interact with nonequilibrium QCD matter in the early stages of heavy-ion collisions. It couples the Improved Opacity Expansion for medium-induced radiation with effective kinetic theory simulations that track the bulk evolution, extracting time-dependent parameters to describe the medium. By analyzing isotropic under- and over-occupied plasmas as well as expanding systems, the work shows that pre-equilibrium dynamics can leave measurable imprints on jet radiation and substructure, particularly through early-time variations of the jet quenching parameter and screening mass. The findings highlight the potential of jets as tomographic probes of early-time QCD dynamics and provide a framework to incorporate pre-equilibrium physics into jet quenching phenomenology, while noting limitations related to bulk anisotropies and plasma instabilities that warrant future study.

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.

Figures (8)

• Figure 1: Summary of results for single gluon production in the presence of an out-of-equilibrium QCD medium, as illustrated on the top left panel. The bulk matter's time evolution (in green) is captured, for a Bjorken expanding system, by dynamical $\hat{q}_0$ and $\mu_*$, shown on the top right panel for a coupling $\lambda =10$. This allows to compute the gluon emission probability in the IOE as illustrated on the bottom left, and to compare to static and thermal matched results. These permit to gauge the relevance of the out-of-equilibrium bulk modifications during the jet evolution. In the remaining panel, we show the corresponding ratio of the spectra's maximum between the out-of-equilibrium and equilibrium scenarios: the lack of convergence to unity at late times points to the importance of the early stages in final state jet observables.
• Figure 2: Time evolution of the bare jet quenching parameter $\hat{q}_0$, and screening mass $\mu_\ast$ for the systems considered in this paper, as extracted from the EKT simulations. In the left column, we show the isotropic systems, where the quantities are depicted in units of $T$ (underoccupied system, top panel) and $Q$ (overoccupied system, bottom panel). In the right column, we show $\hat{q}_0/Q_s^3$ and $\mu_\ast/Q_s$ for the expanding system in units of the saturation momentum $Q_s$ for various couplings.
• Figure 3: Illustration of the jet shape at leading order in $\alpha_s$: the probability of energy deposited in the annulus of radius $r$ and $R$.
• Figure 4: Medium-induced gluon spectrum (left column) for an isotropic initially underoccupied (top row) and overoccupied (bottom row) system. The center and right column show the ratio observables $\chi_i$ defined in Eqs. \ref{['eq:ratio-jetshape']} and \ref{['eq:ratio-maxima']}.
• Figure 5: Medium-induced gluon spectrum (left column) for Bjorken expanding matter. In the middle and right columns, we show the ratio observables $\chi_i$ defined in Eqs. \ref{['eq:ratio-jetshape']} and \ref{['eq:ratio-maxima']}.