Jet measurements in pp, p--Pb and Pb--Pb collisions with ALICE at the LHC

TL;DR

This paper presents a comprehensive analysis of jet production and properties in proton-proton, proton-lead, and lead-lead collisions at the LHC using the ALICE detector, comparing measurements to theoretical models and exploring medium effects.

Contribution

It provides new measurements of jet cross sections, shapes, and fragmentation in various collision systems and compares them to NLO pQCD calculations and MC models, highlighting medium effects in Pb--Pb collisions.

Findings

Jet cross sections match NLO pQCD in pp at 2.76 TeV.

MC models fail to reproduce jet cross sections at 7 TeV.

No significant modification of jets in p--Pb collisions.

Abstract

We present a systematic study of jet measurements in pp, p--Pb and Pb--Pb collisions using the ALICE detector at the LHC. Jet production cross sections are measured in pp collisions at $\sqrt{s}$ = 2.76 and 7~TeV, in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02~TeV and in Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76~TeV. Jet shape observables and fragmentation distributions are measured in pp collisions at 7~TeV. Jets are reconstructed at midrapidity in a wide range of transverse momentum using sequential recombination jet finding algorithms ($k_{\rm T}$, anti-$k_{\rm T}$, and SISCone) with several values of jet resolution parameter $R$ in the range 0.2 -- 0.6. Measurements are compared to Next-to-Leading Order (NLO) perturbative Quantum Chromodynamics (pQCD) calculations and predictions from Monte Carlo (MC) event generators such as PYTHIA, PHOJET and HERWIG. Jet production cross sections are well reproduced by NLO pQCD calculations in pp collisions at $\sqrt{s}$~=~2.76~TeV. MC models could not explain the jet cross sections in pp collisions at $\sqrt{s}$ = 7 TeV, whereas jet shapes and fragmentation distributions are rather well reproduced by these models. The jet nuclear modification factor $R_{\rm pPb}$ in p--Pb collisions is found to be consistent with unity indicating the absence of large modifications of the initial parton distribution or strong final state effects on jet production, whereas a large jet suppression is observed in Pb--Pb central events with respect to peripheral events indicating formation of a dense medium in central Pb--Pb events.