Using $\gamma$+jets to quantify medium-induced jet broadening in heavy-ion collisions

Abstract

The structure of jets produced in high-energy nucleus-nucleus collisions carries information on parton energy loss and interaction in the quark-gluon plasma. This parton energy loss results in migration of jets in terms of transverse momentum, leading to a selection bias in inclusive jet measurements. Using the JEWEL event generator, we investigate a strategy to reduce selection bias and access medium-induced jet broadening, in an experimentally viable way. As a baseline, we first consider large-R $\gamma$+jet events, with little selection bias, which show a clear signal of medium-induced broadening in the jet girth. However, large-R is experimentally inaccessible due to the large underlying event fluctuations in heavy-ion collisions, so we re-cluster a collection of small-radius ($r_{\rm sub}$ = 0.2) jets as a proxy for the large-radius (R = 1.2) jet, which we refer to as a trimmed jet. We quantify to what extent trimmed jets recover signals of jet broadening and investigate its dependence on the subjet cone size and the minimum transverse momentum. We study the internal structure of the subjets to expose the dependence of jet quenching on different subjet configurations, finding a strong narrowing of PbPb jets for the 1-subjet configurations, and a visible signature of medium-induced broadening for sub-leading subjets. The jet radial profile reveals that contributions from medium-induced broadening are distinct from radiation in pp collisions. These contributions are not easily recovered using our trimming procedure, but are substantially enhanced in single-subjet configurations when considering the profile \emph{beyond} the subjet radius. This provides guidance for future experimental studies of jet-medium interactions using $\gamma$+jet in heavy-ion collisions.