Measurements of hard probes of the quark-gluon plasma with the ATLAS experiment at the LHC

Barbara Wosiek

doi:10.1016/j.nuclphysbps.2015.09.250

Abstract

ATLAS results on the production of high-transverse momentum probes in Pb+Pb and p+Pb collisions at the LHC are presented. The focus is on the jet measurements, which provide a useful tool to study the hot, dense and coloured matter created in ultra-relativistic heavy ion collisions. The ATLAS experiment has measured inclusive jet yields in pp and Pb+Pb collisions at sNN=2.76 TeV and in p+Pb collisions at sNN=5.02 TeV. The jet nuclear modification factor, RAA, is shown as a function of jet pT, rapidity and collision centrality. The RAA weakly increases with pT, shows no dependence on rapidity and smoothly decreases with the collision centrality. In 10% of the most central collisions, jet production is suppressed by a factor of two relative to pp yields scaled by the number of binary nucleon-nucleon collisions. Charged-particle fragmentation functions of jets are also measured and ratios of fragmentation functions between different centrality intervals show a centrality-dependent modification. The jet measurements in p+Pb collisions show jet enhancement in peripheral collisions and suppression in central collisions as compared to the scaled pp jet yields. These modifications are most pronounced at forward rapidities and at large jet transverse momenta. Jet production at forward rapidities shows a scaling in the total jet energy, suggesting that the modification of jet production in p+Pb collisions may depend on the kinematics of the initial hard parton-parton scattering.

Highlights

In ultra-relativistic heavy ion collisions, a medium of the dense matter, composed of deconfined quarks and gluons, is produced
To the jet spectrum measured in pp collisions, the phenomenon known as the jet quenching
The centrality of Pb+Pb collisions is characterized by ΣETFCal, the total transverse energy measured in the ATLAS forward calorimeters covering 3.2 < |η| < 4.9

Summary

Introduction

In ultra-relativistic heavy ion collisions, a medium of the dense matter, composed of deconfined quarks and gluons, is produced. The geometric scaling hypothesis can be tested with measurements of probes insensitive to medium properties Such a test provide electroweak bosons decaying into colour neutral particles. The yields of photons as well as W and Z bosons decaying leptonically and measured in Pb+Pb collisions at the LHC scale, as expected, with the number of binary nucleon-nucleon collisions [2, 3, 4] This confirms that the geometric enhancement in the yield of the high-pT probe in more central collisions, which is due to the larger overlap, is correctly accounted for by scaling the per-event yield in a given centrality interval by the average per-collision flux of nucleons, TAA , or equivalently, the mean number of nucleon-nucleon collisions, Ncoll. For data-taking, ATLAS used either a minimum bias or the jet trigger [6], providing high statistics samples of jets

Inclusive jet spectra

Fragmentation of the quenched jets

Findings

Summary and discussion