Elsevier

Nuclear Physics A

Volume 931, November 2014, Pages 382-387
Nuclear Physics A

Measurement of jet pT spectra and RAA in pp and Pb–Pb collisions at sNN=2.76 TeV with the ALICE detector

https://doi.org/10.1016/j.nuclphysa.2014.08.033Get rights and content

Abstract

Hard-scattered partons provide an ideal probe for the study of the Quark–Gluon Plasma because they are produced prior to the formation of the QCD medium in heavy-ion collisions. Jet production is therefore susceptible to modifications induced by the presence of the medium (“jet quenching”). Both RHIC and LHC experiments have provided compelling evidence of jet quenching. Jets are reconstructed in ALICE utilizing the central tracking system for the charged constituents and the Electromagnetic Calorimeter for the neutral constituents. Jet spectra are reported for central (0–10%) and semi-central (10–30%) Pb–Pb events at sNN=2.76 TeV. The nuclear modification factor, determined using a pp baseline measured at the same collisional energy, shows a strong suppression of jet production in central Pb–Pb collisions with the expected centrality ordering. Observations are in qualitative agreement with medium-induced energy loss models. Furthermore, indication of a path-length dependence of jet suppression is inferred from measurements of the yields relative to the orientation of the event plane.

Introduction

The study of jets in ultra-relativistic heavy-ion collisions is intimately connected to the investigation of the properties of the Quark–Gluon Plasma (QGP). The QGP is a deconfined state of matter, in which the relevant degrees of freedom are those of strongly-interacting quarks and gluons. Simple quantum-mechanical considerations, based on the Heisenberg principle of indetermination, imply that hard-scattering processes, with a large momentum transfer, happen at a much smaller time scale as compared to that of the QGP formation, which is driven by many low momentum scatterings. The subsequent transport of the hard scattered parton through the medium and its fragmentation are expected to be considerably modified through elastic collisions and medium-induced radiation [1]. These phenomena are usually referred to as “jet quenching”. Measurements at both RHIC [2], [3] and the LHC [4], [5], [6] have confirmed these predictions using a variety of observables. Jet reconstruction can take advantage of the more accurate estimate of the energy of the parton compared to high transverse momentum (pT) single hadron measurements, which are often used as proxies for jets.

In these proceedings we report measurements of the jet nuclear modification factor and charged jet v2 performed by the ALICE experiment for Pb–Pb collisions at sNN=2.76 TeV. These results are based on data collected by ALICE in 2011 and extend previous measurements reported in Refs. [7], [8].

Section snippets

Experimental setup and analysis techniques

For a complete description of the ALICE detector and its performance see Refs. [10] and [11], respectively. The main sub-detectors used in the present measurement are the VZERO, the central tracking system and the Electromagnetic Calorimeter (EMCal). The VZERO detector consists of segmented scintillators covering the full azimuth at forward rapidity. It is used to measure the centrality of the Pb–Pb events and also provides the set of minimum bias and centrality triggers used to collect the

Results

Fig. 2(a) shows the anti-kT R=0.2 jet pT spectra at mid-rapidity for the 0–10% and the 10–30% centrality classes. In order to compare with the same measurement performed in pp collisions at the same sNN [14], the jet yield in Pb–Pb has been divided by the number of binary collisions calculated in a Monte Carlo Glauber model [9] that assumes independent binary nucleon–nucleon collisions. The systematic uncertainty is dominated by the tracking efficiency uncertainty and the unfolding

Conclusions

In these proceedings we have reported new measurements of jet suppression performed by the ALICE experiment. The measured suppression for both central and semi-central events is in qualitative agreement with both the YaJEM and JEWEL models. In order to obtain a tighter constraint on the models the path-length dependence of the parton energy loss has been explored by measuring the charged jet v2, which shows hints of such an effect for semi-central events. ALICE has also measured jet yields in

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    A list of members of the ALICE Collaboration and acknowledgments can be found at the end of this issue.

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