Measurement of charged jet suppression in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV

TL;DR

This work measures inclusive charged-jet spectra in Pb–Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV to study jet quenching, reconstructing jets with anti-$k_{\mathrm{T}}$ for $R=0.2$ and $R=0.3$ from tracks down to $p_T=0.15$ GeV/$c$. Background subtraction and an unfolding procedure correct for event-by-event fluctuations and detector effects, enabling a direct comparison of jet yields across centralities via $R_{CP}$ and to $pp$-like expectations. The results show strong suppression in central events, with $0.3< R_{CP} < 0.5$, similar to charged-hadron suppression, and no significant broadening of the jet radial structure for $R<0.3$; this implies substantial energy transport to angles larger than $R=0.3$. By contrasting unbiased and leading-track biased jets, and comparing $R=0.2$ versus $R=0.3$, the study finds little evidence for modification of the hard jet core or significant radial broadening within $R<0.3$, suggesting that medium-induced radiation largely lies outside the jet cone or is redistributed at large angles. Overall, the analysis provides tight constraints on jet-quenching dynamics and informs models of parton energy loss in the QGP.

Abstract

A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV is reported. Jets are reconstructed from charged particles using the anti-$k_{\rm T}$ jet algorithm with jet resolution parameters $R$ of $0.2$ and $0.3$ in pseudo-rapidity $|η|<0.5$. The transverse momentum $p_{\rm T}$ of charged particles is measured down to $0.15$ GeV/$c$ which gives access to the low $p_{\rm T}$ fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter $R=0.3$ considered in the analysis. The fragmentation bias introduced by selecting jets with a high $p_{\rm T}$ leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with $R=0.2$ and $R=0.3$ is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with $R<0.3$.

Figures (12)

• Figure 1: Uncorrected jet spectra after background subtraction, with radius parameters $R=0.2$ (left) and $R=0.3$ (right) in central Pb--Pb events, without leading particle selection ($unbiased$, black circles) and with at least one particle with $p_{\mathrm{T}} > 5$ (green crosses) or $10$ GeV/$c$ (red squares).
• Figure 2: Left: ${\mathrm{\delta}}p_{\mathrm{T}}^{{\mathrm{ch}}}$ distribution for jets with resolution parameter $R=0.2$ and $R=0.3$ measured with random cones in central collisions. Right: Width of the background fluctuation ${\mathrm{\delta}}p_{\mathrm{T}}^{{\mathrm{ch}}}$ distribution as a function of centrality for cone radii $R=0.2$ and $R=0.3$. The shaded uncertainty bands indicate the difference between the width of the ${\mathrm{\delta}}p_{\mathrm{T}}^{{\mathrm{ch}}}$ distribution from random cones and high $p_{\mathrm{T}}$ probe embedding.
• Figure 3: Distributions of relative transverse momentum difference between detector and particle level anti-$k_\mathrm{T}$ jets with $R=0.2$ and $R=0.3$ and several ranges of jet transverse momentum at particle level. The distributions correspond to the 10% most central events. Events were generated using PYTHIA with the standard ALICE detector response simulation using GEANT3 and the data reconstruction algorithms and settings used for Pb--Pb events. The dominant systematic uncertainty is the uncertainty on tracking efficiency.
• Figure 4: Jet detector response for jet finding resolution parameter $R=0.3$ for the 10% most central events. Data points extracted from event and full detector simulation. Systematic uncertainty originates from the uncertainty on the tracking efficiency. Left: mean of the jet response for charged jets with $R=0.3$. See text for details. Right: Mean, most probably value and quartiles of the jet response as a function of jet momentum.
• Figure 5: Jet-finding efficiency for inclusive unbiased and leading track biased jets extracted from event and detector simulation for the 10% most central events. Left panel: $R=0.2$. Right panel: $R=0.3$.