Abstract
We report the measurement of a new observable of jet quenching in central Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV, based on the semi-inclusive rate of charged jets recoiling from a high transverse momentum (high-$p_{\rm T}$) charged hadron trigger. Jets are measured using collinear-safe jet reconstruction with infrared cutoff for jet constituents of 0.15 GeV/$c$, for jet resolution parameters $R = 0.2$, 0.4 and 0.5. Underlying event background is corrected at the event-ensemble level, without imposing bias on the jet population. Recoil jet spectra are reported in the range $20<p_\mathrm{T,jet}^\mathrm{ch}<100$ GeV/$c$. Reference distributions for pp collisions at $\sqrt{s} = 2.76$ TeV are calculated using Monte Carlo and NLO pQCD methods, which are validated by comparing with measurements in pp collisions at $\sqrt{s} = 7$ TeV. The recoil jet yield in central Pb-Pb collisions is found to be suppressed relative to that in pp collisions. No significant medium-induced broadening of the intra-jet energy profile is observed within 0.5 radians relative to the recoil jet axis. The angular distribution of the recoil jet yield relative to the trigger axis is found to be similar in central Pb-Pb and pp collisions, with no significant medium-induced acoplanarity observed. Large-angle jet deflection, which may provide a direct probe of the nature of the quasi-particles in hot QCD matter, is explored.
Highlights
Hadronic jets are unique probes of the hot Quantum Chromodynamic (QCD) matter generated in nuclear collisions at collider energies
We have reported measurements of jet quenching in central Pb-Pb collisions at the LHC, using a new analysis method based on the semi-inclusive distribution of jets recoiling from a high-pT trigger hadron
Discrimination of coincident jet yield from background is carried out at the level of ensemble-averaged distributions with a trigger-difference technique, with no selection bias imposed on the recoil jet population
Summary
Hadronic jets are unique probes of the hot Quantum Chromodynamic (QCD) matter generated in nuclear collisions at collider energies. The rate of large angular deviations is measured; this rate may be dominated by single hard (Moliere) scattering, which could potentially probe the quasi-particle nature of the hot QCD medium [3, 28] These observables are directly comparable to theoretical calculations, without the need to model the heavy-ion collision background, due to utilization of a hadron trigger, the semiinclusive nature of the observables, and the background suppression technique. The Pb-Pb collision data were recorded during the 2011 LHC Pb-Pb run at sNN = 2.76 TeV This analysis uses the 0–10% most-central Pb-Pb collisions selected by the online trigger based on the hit multiplicity measured in the forward V0 detectors.
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