Centrality evolution of the charged–particle pseudorapidity density over a broad pseudorapidity range in Pb–Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV

The centrality dependence of the charged–particle pseudorapidity density measured with ALICE in Pb–Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV over a broad pseudorapidity range is presented. This Letter extends to more peripheral collisions the previous results reported by ALICE. No strong evolution of the charged–particle pseudorapidity density distributions with centrality is observed, and when the results are normalized to the number of participating nucleons in the collisions, the centrality evolution is likewise small. This suggests that hard contributions to the charged–particle multiplicity are limited. The broad pseudorapidity range allows precise estimates of the total number of produced charged particles which we find to range from $162\pm 22$(syst.) to $17170\pm 770$(syst.) in 80–90% and 0–5% central collisions, respectively. The results are compared to models which describe ${\rm d}N_{\rm ch}/{\rm d}\eta$ at mid–rapidity in the most central Pb–Pb collisions and it is found that these models do not capture all features of the distributions. In the forward regions where the signal is dominated by secondary particles produced in the surrounding material, we use a data-driven correction to extract the primary charged–particle density.