Abstract
The transverse momentum (pT) spectrum and nuclear modification factor (RAA) of reconstructed jets in 0–10% and 10–30% central Pb–Pb collisions at sNN=2.76 TeV were measured. Jets were reconstructed using the anti-kT jet algorithm with a resolution parameter of R=0.2 from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal). The jet pT spectra are reported in the pseudorapidity interval of |ηjet|<0.5 for 40<pT,jet<120 GeV/c in 0–10% and for 30<pT,jet<100 GeV/c in 10–30% collisions. Reconstructed jets were required to contain a leading charged particle with pT>5 GeV/c to suppress jets constructed from the combinatorial background in Pb–Pb collisions. The leading charged particle requirement applied to jet spectra both in pp and Pb–Pb collisions had a negligible effect on the RAA. The nuclear modification factor RAA was found to be 0.28±0.04 in 0–10% and 0.35±0.04 in 10–30% collisions, independent of pT,jet within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching.
Highlights
Discrete formulations of Quantum Chromodynamics (QCD) predict the existence of a cross-over transition from normal nuclear matter to a new state of matter called the Quark–Gluon Plasma (QGP), where the partonic constituents, quarks and gluons, are deconfined
The transverse momentum spectrum and nuclear modification factor (RAA) of jets reconstructed from charged particles measured by the ALICE tracking system and neutral energy measured by the ALICE Electromagnetic Calorimeter are measured with
The jets were required to contain at least one charged particle with producing high transverse momentum (pT) > 5 GeV/c. The effect of this requirement on the reported RAA was evaluated by the ratios of the jet spectra with the 5 GeV/c to no requirement compared to expectations on PYTHIA, and found not to have an observable effect within the uncertainties of the measurement
Summary
Discrete formulations of Quantum Chromodynamics (QCD) predict the existence of a cross-over transition from normal nuclear matter to a new state of matter called the Quark–Gluon Plasma (QGP), where the partonic constituents, quarks and gluons, are deconfined. The QGP state is expected to exist at energy densities above 0.5 GeV/fm and temperatures above 160 MeV [1], which can be reached in collisions of heavy-ions at ultra-relativistic energies. Hard scattering is expected to occur early in the collision evolution, producing high transverse momentum (pT) partons, which propagate through the expanding medium and eventually fragment into jets of hadrons. The production cross section of the initial hard scattered partons is calculable using perturbative QCD (pQCD), and the contribution from the nonperturbative hadronization can be well calibrated via jet measurements in pp collisions
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