Abstract
Measurements of dijet pT correlations in Pb+Pb and pp collisions at a nucleon–nucleon centre-of-mass energy of sNN=2.76 TeV are presented. The measurements are performed with the ATLAS detector at the Large Hadron Collider using Pb+Pb and pp data samples corresponding to integrated luminosities of 0.14 nb−1 and 4.0 pb−1, respectively. Jets are reconstructed using the anti-kt algorithm with radius parameter values R=0.3 and R=0.4. A background subtraction procedure is applied to correct the jets for the large underlying event present in Pb+Pb collisions. The leading and sub-leading jet transverse momenta are denoted pT1 and pT2. An unfolding procedure is applied to the two-dimensional (pT1,pT2) distributions to account for experimental effects in the measurement of both jets. Distributions of (1/N)dN/dxJ, where xJ=pT2/pT1, are presented as a function of pT1 and collision centrality. The distributions are found to be similar in peripheral Pb+Pb collisions and pp collisions, but highly modified in central Pb+Pb collisions. Similar features are present in both the R=0.3 and R=0.4 results, indicating that the effects of the underlying event are properly accounted for in the measurement. The results are qualitatively consistent with expectations from partonic energy loss models.
Highlights
Jets have long been considered an important tool for studying the matter produced in ultra-relativistic heavy-ion collisions
Shown are the corresponding distributions obtained from the Pythia 6 sample used in the Mo√nte Carlo (MC) studies and from Pythia 8 using the AU2 tune and Herwig++ [38] with the underlying event (UE)-EE-3 [39] tune
An additional sample, referred to as Powheg+Pythia 8 is generated using Powheg-Box 2.0 [40,41,42], which is accurate to next-to-leading order in perturbative QCD, and interfaced with Pythia 8 to provide a description of the parton shower and hadronisation
Summary
Jets have long been considered an important tool for studying the matter produced in ultra-relativistic heavy-ion collisions. Jets produced in the initial stage of the collision lose energy as they propagate through the medium This phenomenon, known as jet quenching, was first observed at the Relativistic Heavy Ion Collider (RHIC) [1,2]. In Pb+Pb collisions the transverse momentum (pT) balance between two jets was found to be distorted, resulting from configurations in which the two jets suffer different amounts of energy loss This measurement was the experimental confirmation of some of the initial pictures of jet quenching and signatures of a deconfined medium [4].
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