Suppression of high pT hadrons in Pb+Pb collisions at LHC

Hadron production and their suppression in Pb + Pb collisions at LHC at a center-of-mass energy of root s(NN) = 2.76 TeV are studied within a multiphase transport (AMPT) model whose initial conditions are obtained from the recently updated HIJING 2.0 model. The centrality dependence of charged hadron multiplicity dN(ch)/d(eta) at midrapidity was found quite sensitive to the largely uncertain gluon shadowing parameter s(g) that determines the nuclear modification of the gluon distribution. We find final-state parton scatterings reduce considerably hadron yield at midrapidity and enforces a smaller gluon shadowing to be consistent with dN(ch)/d(eta) data at LHC. With such a constrained parton shadowing. charged hadron and neutral pion production over a wide transverse momenta range are investigated in AMPT. Relative to nucleon-nucleon collisions, the particle yield in central heavy ion collisions is suppressed due to parton energy loss. While the calculated magnitude and pattern of suppression is found consistent with that measured in Au + Au collisions at root s(NN) = 0.2 TeV at RHIC, at the LHC energy the suppression is overpredicted which may imply the medium formed at LHC is less opaque than expected from simple RHIC extrapolations. Reduction of the QCD coupling constant alpha(s) by similar to 30% in the higher temperature plasma formed at LHC as compared to that at RHIC was found to reproduce the measured suppression at LHC. (C) 2012 Elsevier B.V. All rights reserved.