UC Berkeley

This paper presents the first measurement of the angle between different jet axes (denoted as ) in collisions. The measurement is carried out in the 0–10 % most-central events at TeV. Jets are assembled by clustering charged particles at midrapidity using the algorithm with resolution parameters and 0.4 and transverse momenta in the intervals and , 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 distribution relative to can be explained if quark-initiated jets are more likely to emerge from the medium than gluon-initiated jets. These new measurements disfavor intrajet broadening as implemented in a simple model calculation with the Baier-Dokshitzer-Mueller-Peigne-Schiff formalism for energy loss in the QGP. The comparison of and collisions shows sensitivity to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss.