Jet production in pp, p–Pb and Pb–Pb collisions measured by ALICE

Abstract

Particle jets, formed when a hard scattered parton fragments into a jet of hadrons, are an ideal probe of the medium formed in heavy-ion collisions. The hard-scattered partons that produce them come from early in the collision, prior to the medium formation. These partons lose energy as they traverse the medium, and eventually fragment into jets of hadrons, which exhibit a modification when compared to jets produced in pp and p-Pb collisions. At LHC energies, the parton production cross-section is much larger than at RHIC, allowing jets to be reconstructed over a much wider kinematic range. Jet reconstruction allows for a more differential investigation of the parton energy loss than single hadrons, which have been used as jet proxies in the past, as the jets collect a larger percentage of the final state energy, which means their kinematics are more closely correlated to the kinematics of the initial parton.Jets are reconstructed in ALICE either using information from the tracking systems, or by combining this with the ALICE electromagnetic calorimeter (EMCal). In these proceedings, jet spectra from 2.76 TeV Pb-Pb and pp collisions will be presented. In particular, the centrality and event-plane dependence of the measured spectra and the background will be discussed. Jets from different centrality classes and event-plane orientations provide additional information necessary for understanding the path-length and temperature dependence of energy loss mechanisms. The reconstruction and correction procedures for jets will be shown. Results from Pb-Pb events will be compared to the baseline pp and p-Pb results, which allows the initial state and cold nuclear matter effects to be disentangled from hot medium effects. The jet nuclear modification, which quantifies the suppression, will be compared to energy-loss models.