Gluon shadowing and hadron production in heavy-ion collisions at LHC

The recently published first measurement of charged hadron multiplicity density at mid-rapidity $dN_{ch}/d\eta=1584 \pm 4 ({\rm stat.}) \pm 76 ({\rm sys.})$ in central $Pb+Pb$ collisions at $\sqrt{s} = 2.76$ TeV by the ALICE Experiment at LHC is in good agreement with the HIJING2.0 prediction within the experimental errors and theoretical uncertainties. The new data point is used to carry out a combined fit together with the RHIC data to reduce the uncertainty in the gluon shadowing parameter $s_{g}$ which controls the overall magnitude of gluon shadowing at small fractional momentum $x$ in HIJING2.0 model. Predictions on the centrality dependence of charged hadron multiplicity density at mid-rapidity with reduced uncertainty are given for $Pb+Pb$ collisions at $\sqrt{s}=2.76$ and 5.5 TeV. The centrality dependence is surprisingly independent of the colliding energy as compared to RHIC energy for most of centralities starting at $N_{\rm part}=50$ (100) at $\sqrt{s}=2.76$ (7) TeV. However, it becomes stronger in peripheral collisions at higher colliding energies.