Observation and studies of jet quenching in PbPb collisions atsNN=2.76TeV

Abstract

Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV was studied with the Compact Muon Solenoid (CMS) detector at the LHC, using a data sample corresponding to an integrated luminosity of 6.7 μb −1 . Jets are reconstructed using the energy deposited in the CMS calorimeters and studied as a function of collision centrality. With increasing collision centrality, a striking imbalance in dijet transverse momentum is observed, consistent with jet quenching. The observed effect extends from the lower cutoff used in this study (jet p T = 120 GeV/c) up to the statistical limit of the available data sample (jet p T ≈ 210 GeV/c). Correlations of charged particle tracks with jets indicate that the momentum imbalance is accompanied by a softening of the fragmentation pattern of the second most energetic, away-side jet. The dijet momentum balance is recovered when integrating low transverse momentum particles distributed over a wide angular range relative to the direction of the away-side jet.

Jet shape and fragmentation function measurements with CMS

“…In the most central PbPbcollisions (10-30% and 0-10%), an excess of transverse momentum fraction emitted at large radius r > 0.2 emerges, indicating a moderate broadening of the jets in the medium. These observations are consistent with previous studies in CMS which find that the energy that the jets lose in the medium is redistributed at large distances from the jet axis outside the jet cone [10].…”

Interference effects in medium-induced gluon radiation

“…One of the most striking effects of this kind is the large di-jet asymmetry observed in Pb+Pb collisions at the LHC, as reported by the ATLAS [1] and CMS [2] collaborations (see also [3] for related results at RHIC). These data imply that, as a consequence of the interactions between the jet and the medium, the jet energy is transported to larger angles and redistributed into softer fragments as compared to the p+p baseline.…”

Novel subjet observables for jet quenching in heavy-ion collisions

“…Considerations of the RSD distributions show that the R IQR[ΔS 12 ] carries the largest discrimination power for highp T jets, although there is an associated uncertainty of [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]% due to hadronization effects. For low-p T jets ( p T 120 GeV/c) and/or recoil-jets in dijet systems the use of the R Q 1 [ΔS 12 ] and/or the median R Q 2 [ΔS 12 ] may prove more advantageous, with hadronization uncertainties that are smaller than 5% for the chosen subjet parameters.…”

“…Experiments at RHIC and the LHC observed a strong suppression of high transverse momentum particle yields [9][10][11][12][13][14][15], suppression of inclusive and semi-inclusive yields of fully reconstructed jets [16][17][18][19][20], and, more recently, the internal structure of the jets [21][22][23][24][25] for detailed studies of jet quenching. However, in all these measurements the treatment of the background originating from the copiously produced particles not associated to hard scatterings poses an experimental challenge for precise and unbiased measurements.…”