  Hadron production and their suppression in Pb+Pb collisions at LHC at a
center-of-mass energy of 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/deta 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/deta 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 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 ~30% in the
higher temperature plasma formed at LHC as compared to that at RHIC was found
to reproduce the measured suppression at LHC.
