Measurement of the angle between jet axes in Pb-Pb collisions at √𝑠�𝑁�⁢𝑁�=5.02 TeV

Abstract

This paper presents the first measurement of the angle between different jet axes (denoted as Raxis) in Pb-Pb collisions. The measurement is carried out in the 0–10 % most-central events at √sNN = 5.02 TeV. Jets are assembled by clustering charged particles at midrapidity using the anti-kT algorithm with resolution parameters R = 0.2 and 0.4 and transverse momenta in the intervals 40 < p ch jet T < 140 GeV/c and 80 < p ch jet T < 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 suggests 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 disfavor intrajet pT broadening as implemented in a simple model calculation with the Baier-Dokshitzer-Mueller-PeigneSchiff formalism for energy loss in the QGP. The comparison of Pb-Pb and pp collisions shows sensitivity to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss.