Abstract
Studies of the fragmentation of jets into charged particles in heavy-ion collisions can provide information about the mechanism of jet-quenching by the hot and dense QCD matter created in such collisions, the quark-gluon plasma. This paper presents a measurement of the angular distribution of charged particles around the jet axis in $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV Pb+Pb and $pp$ collisions, using the ATLAS detector at the LHC. The Pb+Pb and $pp$ data sets have integrated luminosities of 0.49 nb$^{-1}$ and 25 pb$^{-1}$, respectively. The measurement is performed for jets reconstructed with the anti-$k_{t}$ algorithm with radius parameter $R = 0.4$ and is extended to an angular distance of $r= 0.8$ from the jet axis. Results are presented as a function of Pb+Pb collision centrality and distance from the jet axis for charged particles with transverse momenta in the 1$-$63 GeV range, matched to jets with transverse momenta in the 126$-$316 GeV range and an absolute value of jet rapidity of less than 1.7. Modifications to the measured distributions are quantified by taking a ratio to the measurements in $pp$ collisions. Yields of charged particles with transverse momenta below 4 GeV are observed to be increasingly enhanced as a function of angular distance from the jet axis, reaching a maximum at $r=0.6$. Charged particles with transverse momenta above 4 GeV have an enhanced yield in Pb+Pb collisions in the jet core for angular distances up to $r = 0.05$ from the jet axis, with a suppression at larger distances.
Highlights
Ultrarelativistic nuclear collisions at the Large Hadron Collider (LHC) produce hot, dense matter called a quarkgluon plasma, QGP
Jets with large transverse momenta, pjTet, in central leadlead (Pb + Pb) collisions at the LHC are measured at approximately half the rates in pp collisions when the nuclear overlap function of Pb + Pb collisions is taken into account [3,4,5,6,7]
Particles carrying a large fraction of the jet momentum are generally closely aligned with the jet axis, whereas low-momentum particles are observed to have a much broader angular distribution extending outside the jet [9,21,22,23,24]
Summary
Ultrarelativistic nuclear collisions at the Large Hadron Collider (LHC) produce hot, dense matter called a quarkgluon plasma, QGP (see Refs. [1,2] for recent reviews). Back-to-back dijet [8,9,10] and photon-jet pairs [11,12] are observed to have less-balanced transverse momenta in Pb + Pb collisions compared to pp collisions These observations suggest that some of the energy from the hard-scattered parton may be transferred outside of the jet through its interaction with the QGP medium. Particles carrying a large fraction of the jet momentum are generally closely aligned with the jet axis, whereas low-momentum particles are observed to have a much broader angular distribution extending outside the jet [9,21,22,23,24].
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