Modification of charged-particle jets in event-shape engineered Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

TL;DR

This study measures charged-particle jet yields in Pb-Pb collisions at 5.02 TeV, analyzing their dependence on event shape and orientation to understand jet quenching and path-length effects in quark-gluon plasma.

Contribution

It introduces event-shape engineering to investigate the path-length dependence of jet quenching in heavy-ion collisions at the LHC.

Findings

Out-of-plane jets are more suppressed than in-plane jets.

Jet suppression varies with event shape and transverse momentum.

Results support azimuthally anisotropic jet quenching in QGP.

Abstract

Charged-particle jet yields have been measured in semicentral Pb$-$Pb collisions at center-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE detector at the LHC. These yields are reported as a function of the jet transverse momentum, and further classified by their angle with respect to the event plane and the event shape, characterized by ellipticity, in an effort to study the path-length dependence of jet quenching. Jets were reconstructed at midrapidity from charged-particle tracks using the anti-$k_{\rm T}$ algorithm with resolution parameters $R =$ 0.2 and 0.4, with event-plane angle and event-shape values determined using information from forward scintillating detectors. The results presented in this letter show that, in semicentral Pb$-$Pb collisions, there is no significant difference between jet yields in predominantly isotropic and elliptical events. However, out-of-plane jets are observed to be more suppressed than in-plane jets. Further, this relative suppression is greater for low transverse momentum ($<$ 50 GeV/$c$) $R =$ 0.2 jets produced in elliptical events, with out-of-plane to in-plane jet-yield ratios varying up to 5.2$\sigma$ between different event-shape classes. These results agree with previous studies indicating that jets experience azimuthally anisotropic suppression when traversing the QGP medium, and can provide additional constraints on the path-length dependence of jet energy loss.