Abstract
Electromagnetic probes such as photons do not participate in the strong interaction, and thus provide a clean measurement of the initial state in nuclear collisions. Correlations of photons balancing with jets in PbPb collisions constitute the golden channel to study parton energy loss in strongly interacting matter, since the photon not only determines the initial transverse momentum of the balancing parton, but also preferentially selects quark jets. We will present new results from pp and PbPb collisions at 5.02 TeV collision energy, using the high statistics data collected with the CMS detector in the 2015 LHC run. The results include detailed studies of azimuthal and momentum correlations of isolated photons and associated jets, as well as jet IAA, as a function of photon pT and collision centrality.
Highlights
In heavy ion collusions at ultrarelativistic energies, quarks and gluons can exist freely as part of a hot, dense medium [1]
Photon + jet events in PbPb and pp collisions are used to quantify absolute parton energy loss in the medium, by studying the azimuthal angle between photon and correlated jets, the ratio of jet transverse momentum with photon transverse momentum for pairs back-to-back in azimuth, and the average number of jets correlated to a photon
The most relevant subdetector systems employed in this analysis are the electromagnetic calorimeter (ECAL) and the hadronic calorimeter (HCAL)
Summary
In heavy ion collusions at ultrarelativistic energies, quarks and gluons can exist freely as part of a hot, dense medium [1]. To study this medium, referred to as the Quark-Gluon Plasma, or QGP, energetic partons are a useful tool. These probes are produced before medium formation, and modifications relative to vacuum can be interpreted as resulting from interaction with the medium as the parton travels through it. By studying photon + jet momentum imbalance in the medium and comparing to reference vacuum, absolute energy loss of partons can be quantified. Photon + jet events in PbPb and pp collisions are used to quantify absolute parton energy loss in the medium, by studying the azimuthal angle between photon and correlated jets, the ratio of jet transverse momentum with photon transverse momentum for pairs back-to-back in azimuth, and the average number of jets correlated to a photon
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