Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions atsNN=2.76  TeVwith the ATLAS Detector at the LHC

Abstract

By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

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.…”

Angular distributions of the quenched energy flow from dijets with different radius parameters in CMS

“…This has been observed at the LHC via dijet momentum asymmetry (A J = (p T,1 − p T,2 )/(p T,1 + p T,2 )) modification in PbPb relative to pp collisions [1]. The observed lost momentum can be recovered by summing softer particles through large angular space η-φ.…”

Measurement of the D0 Meson Production in Pb–Pb and p–Pb Collisions

“…"1" and "2" refer to the leading and subleading jet, respectively, was performed by ATLAS [60] and CMS [61] at the LHC. The most striking observation is the large centrality-dependent increase of the imbalance in the energy of the two jets as reported in Figure 2.6.…”