Differential measurements of jet substructure and partonic energy loss in Au$+$Au collisions at $\sqrt{s_{\rm{NN}}} =200$ GeV

TL;DR

This paper measures jet substructure in proton-proton and gold-gold collisions at 200 GeV, finding that high-energy jets show vacuum-like properties and no significant medium modifications, suggesting soft gluon radiation dominates energy loss.

Contribution

First measurements of subjet observables, including the novel opening angle sj, in heavy-ion collisions, providing new insights into jet quenching and medium effects at RHIC energies.

Findings

Jet substructure observables are more robust to background than traditional measures.

No significant modifications of jet substructure in high-energy di-jet pairs in Au+Au collisions.

Energy loss appears consistent with soft medium-induced gluon radiation from a single color charge.

Abstract

The STAR collaboration presents jet substructure measurements related to both the momentum fraction and the opening angle within jets in \pp and \AuAu collisions at \sqrtsn $= 200$ GeV. The substructure observables include SoftDrop groomed momentum fraction (\zg), groomed jet radius (\rg), and subjet momentum fraction (\zsj) and opening angle (\tsj). The latter observable is introduced for the first time. Fully corrected subjet measurements are presented for \pp collisions and are compared to leading order Monte Carlo models. The subjet \tsj~distributions reflect the jets leading opening angle and are utilized as a proxy for the resolution scale of the medium in \AuAu collisions. We compare data from \AuAu collisions to those from \pp which are embedded in minimum-bias \AuAu events in order to include the effects of detector smearing and the heavy-ion collision underlying event. The subjet observables are shown to be more robust to the background than \zg~and \rg. We observe no significant modifications of the subjet observables within the two highest-energy, back-to-back jets, resulting in a distribution of opening angles and the splittings that are vacuum-like. We also report measurements of the differential di-jet momentum imbalance ($A_{\rm{J}}$) for jets of varying \tsj. We find no qualitative differences in energy loss signatures for varying angular scales in the range $0.1 < $ \tsj $ < 0.3$, leading to the possible interpretation that energy loss in this population of high momentum di-jet pairs, is due to soft medium-induced gluon radiation from a single color-charge as it traverses the medium.