Measurement of the angle between jet axes in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

TL;DR

This study measures the angle between different jet axes in lead-lead collisions at 5.02 TeV, revealing insights into quark-gluon plasma effects and jet quenching mechanisms.

Contribution

First measurement of jet axis angle differences in Pb-Pb collisions, providing new constraints on jet quenching models and QGP properties.

Findings

Pb-Pb jet axis distribution narrows compared to pp

Discards intra-jet pT broadening as main energy loss mechanism

Data favor incoherent energy loss models in QGP

Abstract

This letter presents the first measurement of the angle between different jet axes (denoted as ${\Delta}R$) in Pb$-$Pb collisions. The measurement is carried out in the 0$-$10% most-central events at $\sqrt{s_{\rm NN}} = 5.02$ TeV. Jets are assembled by clustering charged particles at midrapidity using the anti-$k_{\rm T}$ algorithm with resolution parameters $R=0.2$ and $0.4$ and transverse momenta in the intervals $40 < p_{\rm T}^{\rm ch jet} < 140$ GeV/$c$ and $80 < p_{\rm T}^{\rm ch jet} < 140$ GeV/$c$, respectively. Measurements at these low transverse momenta enhance the sensitivity to quark$-$gluon plasma (QGP) effects. A comparison to models implementing various mechanisms of jet energy loss in the QGP shows that the observed narrowing of the Pb$-$Pb distribution relative to pp can be explained if quark-initiated jets are more likely to emerge from the medium than gluon-initiated jets. These new measurements discard intra-jet $p_{\rm T}$ broadening as described in a model calculation with the BDMPS formalism as the main mechanism of energy loss in the QGP. The data are sensitive to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss.