Abstract
Measurements of the centrality and rapidity dependence of inclusive jet production in sNN=5.02 TeV proton–lead (p+Pb) collisions and the jet cross-section in s=2.76 TeV proton–proton collisions are presented. These quantities are measured in datasets corresponding to an integrated luminosity of 27.8 nb−1 and 4.0 pb−1, respectively, recorded with the ATLAS detector at the Large Hadron Collider in 2013. The p+Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval −4.9<η<−3.2 in the direction of the lead beam. Results are presented for the double-differential per-collision yields as a function of jet rapidity and transverse momentum (pT) for minimum-bias and centrality-selected p+Pb collisions, and are compared to the jet rate from the geometric expectation. The total jet yield in minimum-bias events is slightly enhanced above the expectation in a pT-dependent manner but is consistent with the expectation within uncertainties. The ratios of jet spectra from different centrality selections show a strong modification of jet production at all pT at forward rapidities and for large pT at mid-rapidity, which manifests as a suppression of the jet yield in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton–nucleus collisions. Furthermore, the modifications at forward rapidities are found to be a function of the total jet energy only, implying that the violations may have a simple dependence on the hard parton–parton kinematics.
Highlights
Proton–lead (p + Pb) collisions at the Large Hadron Collider (LHC) provide an excellent opportunity to study hard scattering processes involving a nuclear target [1]
The calorimeter system consists of a liquid argon (LAr) electromagnetic (EM) calorimeter covering |η| < 3.2, a steel/scintillator sampling hadronic calorimeter cover ing |η| < 1.7, a LAr hadronic calorimeter covering 1.5 < |η| < 3.2, and two LAr electromagnetic and hadronic forward calorimeters (FCal) covering 3.2 < |η| < 4.9
The R pPb values show a slight (≈ 10%) enhancement above one, many bins are consistent with unity within the systematic uncer tainties
Summary
Proton–lead (p + Pb) collisions at the Large Hadron Collider (LHC) provide an excellent opportunity to study hard scattering processes involving a nuclear target [1]. Studies of forward di hadron angular correlations at RHIC showed a much weaker dijet signal in d + Au collisions than in pp collisions [4,5] These effects have been attributed to the saturation of the parton dis tributions in the gold nucleus [6,7,8], to the modification of the nuclear parton distribution function [9], to the higher-twist contri butions to the cross-section enhanced by the forward kinematics of the measurement [10], or to the presence of a large nucleus [11]. The extended kinematic reach of p + Pb measurements at the LHC allows the study of hard scattering processes that produce forward hadrons or jets over a much wider rapidity and transverse momentum range Such measurements can determine whether the factorisation violations observed at RHIC persist at higher energy and, if so, how the resulting modifications vary as a function of particle or jet momentum and rapidity. The results of such mea surements could test the competing descriptions of the RHIC re sults and, more generally, provide new insight into the physics of hard scattering processes involving a nuclear target
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