Measurement of jet quenching with semi-inclusive hadron-jet distributions in central Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

Abstract

Abstract:We report the measurement of a new observable of jet quenching in central Pb-Pb collisions at √ s NN = 2.76 TeV, based on the semi-inclusive rate of charged jets recoiling from a high transverse momentum (high-p T ) charged hadron trigger. Jets are measured using collinear-safe jet reconstruction with infrared cutoff for jet constituents of 0.15 GeV, for jet resolution parameters R = 0.2, 0.4 and 0.5. Underlying event background is corrected at the event-ensemble level, without imposing bias on the jet population. Recoil jet spectra are reported in the range 20 < p ch T,jet < 100 GeV. Reference distributions for pp collisions at √ s = 2.76 TeV are calculated using Monte Carlo and NLO pQCD methods, which are validated by comparing with measurements in pp collisions at √ s = 7 TeV. The recoil jet yield in central Pb-Pb collisions is found to be suppressed relative to that in pp collisions. No significant medium-induced broadening of the intra-jet energy profile is observed within 0.5 radians relative to the recoil jet axis. The angular distribution of the recoil jet yield relative to the trigger axis is found to be similar in central Pb-Pb and pp collisions, with no significant medium-induced acoplanarity observed. Large-angle jet deflection, which may provide a direct probe of the nature of the quasi-particles in hot QCD matter, is explored. [17][18][19][20] and enhancement in the rate of energy-imbalanced back-to-back di-jet pairs [21,22] have also been observed in nuclear collisions. A measurement of event-averaged missing p T suggests that the radiated energy induced by the interaction of an energetic parton with the medium is carried to a significant extent by soft particles at large angles relative to the jet axis [23].The measurement of reconstructed jets over a wide range in jet energy and jet resolution parameter (R) is required for comprehensive understanding of jet quenching in heavy-ion collisions. Such measurements are challenging, however, due to the presence of complex, uncorrelated background to the jet signal, and the need to minimize biases in the selected jet population imposed by background suppression techniques. Multiple, complementary measurement approaches, differing both in instrumentation and in analysis algorithm, are therefore important to elucidate the physics of jet quenching using reconstructed jets.In this article we present a new approach to the measurement of jet quenching, based on the semi-inclusive distribution of charged jets recoiling from a high-p T charged hadron trigger ("h-jet" coincidence) in central (0-10%) Pb-Pb collisions at √ s NN = 2.76 TeV. Jets are reconstructed using charged particle tracks with the k T [24] and anti-k T algorithms [25], with infrared cutoff for tracks p T,const > 0.15 GeV/c. Uncorrelated background to the recoil jet signal is corrected solely at the level of ensemble-averaged distributions, without eventby-event discrimination of jet signal from background, using a technique that exploits the phenomenology of jet production in QCD. The...

Origins of the di-jet asymmetry in heavy-ion collisions

“…The ability to systematically reconstruct jets above the large and fluctuating background present in ultra-relativistic heavy-ion collisions [1] has opened up a versatile path [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] to study the properties of Quark Gluon Plasma (QGP). Jets are sensitive, through the wide range of scales involved in their development, to a variety of properties of the expanding QGP they traverse.…”

Current and future measurements of semi-inclusivehadron+jet distributions with ALICE

“…The background-subtracted hadron+jet azimuthal distribution was measured in Pb-Pb collisions at ALICE with Run 1 data [2]. The rate of large angle scattering showed no deviation from the vacuum expectation within the experimental uncertainties, though this measurement was statistically limited.…”

“…The measurement of the trigger-normalised recoil jet distributions in Pb-Pb collisions indicates that jets lose a significant amount of energy in Pb-Pb collisions, and that this energy is predominantly radiated to angles greater than R = 0.5 [2]. A similar analysis was performed in p-Pb collisions in different event-activity classes, defined using the signal magintude in the V0A detectors of ALICE, to test whether jets are quenched in smaller systems [3].…”

“…ALICE has measured the trigger-normalised semi-inclusive yield of jets recoiling from a trigger hadron 1 N trig d 2 N jet dp ch T,jet dη jet , differential in jet transverse momentum p ch T,jet . A variable ∆ recoil is then defined as the difference between the trigger-normalised recoil jet distributions in 'reference' and 'signal' trigger track p T intervals T T ref and T T sig [2]:…”

Overall momentum balance and redistribution of the lost energy in asymmetric dijet events in 2.76A TeV Pb-Pb collisions with a multiphase transport model

“…As full jets include the contributions from both leading and subleading fragments of the parton showers, they are expected to provide more detailed information than hadronic observables on the interaction between jet and medium. Various full jet observables have been studied in heavy-ion experiments at RHIC and the LHC, e.g., single inclusive full jet spectra [27][28][29][30][31], the transverse momentum asymmetry distributions and angular correlations for dijet and photon-jet events [32][33][34][35], and the internal structures of the full jets [36][37][38]. In order to understand the observed nuclear modifications of full jet production and structure, it is required to develop theoretical models and calculations that include the effect of the medium on both leading and subleading partons of the full jets [39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57].…”