[Paper Review] Jet Results in pp and Pb-Pb Collisions at ALICE

O. Busch|arXiv (Cornell University)|Jun 12, 2013

Particle physics theoretical and experimental studies8 references3 citations

TL;DR

This paper presents ALICE measurements of jet production in pp and Pb-Pb collisions at the LHC, using charged and neutral particles to reconstruct jets with R = 0.2–0.4. It demonstrates strong momentum-dependent jet suppression in central Pb-Pb collisions, with RAA ≈ 0.5 at high pT, and finds no significant jet broadening or energy redistribution, consistent with medium-induced radiation and energy loss in the quark-gluon plasma.

ABSTRACT

We report results on jet production in pp and Pb-Pb collisions at the LHC from the ALICE collaboration. The jet cross section in pp collisions at $\sqrt{s}$=2.76 TeV is presented, as well as the charged particle jet production cross section and measurements of the jet fragmentation and jet shape in pp collisions at $\sqrt{s}$=7 TeV. NLO pQCD calculations and simulations from MC event generators agree well with the data. Measurements of jets with a resolution parameter $R$=0.2 in Pb-Pb collisions at $\sqrt{s}_{NN}$=2.76 TeV show a strong, momentum dependent suppression in central events with respect to pp collisions. The centrality dependence of the suppression of charged particle jets relative to peripheral events is presented. The ratio of jet spectra with $R$=0.2 and $R$=0.3 is found to be similar in pp and Pb-Pb events. The analysis of the semi-inclusive distribution of charged particle jets recoiling from a high-$p_{ m T}$ trigger hadron allows an unbiased measurement of the jet structure for larger cone radii.

Motivation & Objective

• To measure jet production in pp collisions at √s = 2.76 TeV and 7 TeV to test perturbative QCD predictions.
• To study jet quenching in Pb-Pb collisions at √sNN = 2.76 TeV as a probe of the quark-gluon plasma.
• To investigate the centrality dependence of jet suppression and its momentum dependence in heavy-ion collisions.
• To perform unbiased jet structure measurements using semi-inclusive recoil jets with large R, minimizing reconstruction bias.
• To compare jet shape, fragmentation, and spectrum data with NLO pQCD and Monte Carlo generators (PYTHIA, Phojet) for validation.

Proposed method

• Reconstruct jets using the anti-kT algorithm with R = 0.2–0.4 on charged particle tracks (pT > 150 MeV/c) in the ITS and TPC.
• Include neutral particles from the EMCal (ET > 300 MeV, |η| < 0.7) and subtract charged energy to avoid double-counting.
• Correct jet spectra, shapes, and fragmentation distributions using unfolding and bin-by-bin corrections based on detector simulations.
• Subtract background from the underlying event using median pT density from kT clusters, scaled for charged+neutral jet reconstruction.
• Define nuclear modification factor RAA = (d2Njet/dpT,dη)Pb-Pb / (⟨Ncoll⟩ × d2Njetpp/dpT,dη) to quantify jet suppression.
• Use semi-inclusive recoil jet analysis with trigger hadrons to access larger R jets, applying Δrecoil = (1/Ntrig) × (dN/dpT,jet)trig1-2 − c × (1/Ntrig) × (dN/dpT,jet)trig3-4 to isolate correlated jets.

Experimental results

Research questions

• RQ1 How well do NLO pQCD calculations describe jet production in pp collisions at √s = 2.76 TeV and 7 TeV?
• RQ2 To what extent is jet production suppressed in central Pb-Pb collisions compared to pp, and how does this suppression depend on jet pT and centrality?
• RQ3 Is there evidence of jet broadening or energy redistribution in the quark-gluon plasma, as probed by large-R recoil jets?
• RQ4 How do jet shape and fragmentation distributions in pp collisions compare to PYTHIA and Phojet simulations?
• RQ5 Does the ratio of jet spectra for R = 0.2 and R = 0.3 in Pb-Pb indicate structural changes in the jet due to medium effects?

Key findings

• The inclusive jet cross section in pp collisions at √s = 2.76 TeV agrees well with NLO pQCD calculations when hadronization effects are included.
• Jet shape and fragmentation in pp collisions are well described by PYTHIA (Perugia0 and Perugia2011) and Phojet simulations, with the characteristic hump-backed plateau in fragmentation spectra observed.
• In central Pb-Pb collisions (0–10% centrality), the nuclear modification factor RAA for R = 0.2 jets is significantly less than unity, reaching ≈0.5 at high pT (pT > 50 GeV/c), indicating strong jet quenching.
• The centrality dependence of RCP shows increasing suppression with increasing centrality, with RCP values decreasing from peripheral to central collisions.
• The ratio of jet spectra for R = 0.2 to R = 0.3 is consistent with PYTHIA predictions in both pp and Pb-Pb collisions, indicating no significant broadening of the jet core in the medium.
• The semi-inclusive recoil jet analysis with R = 0.4 and low constituent pT threshold (0.15 GeV/c) shows no significant energy redistribution, as ΔIAA^PYTHIA is consistent across R and pT thresholds.