UC Berkeley

The pseudorapidity density and multiplicity distribution of charged particles produced in proton–proton collisions at the LHC, at a centre-of-mass energy $$\sqrt{s}=7$$ TeV, were measured in the central pseudorapidity region |η|<1. Comparisons are made with previous measurements at $$\sqrt{s}=0.9$$ TeV and 2.36 TeV. At $$\sqrt{s}=7$$ TeV, for events with at least one charged particle in |η|<1, we obtain $$\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta=6.01\pm0.01(\mathrm {stat.})^{+0.20}_{-0.12}(\mathrm{syst.})$$. This corresponds to an increase of $$57.6\%\pm0.4\%(\mathrm{stat.})^{+3.6}_{-1.8}\%(\mathrm{syst.})$$ relative to collisions at 0.9 TeV, significantly higher than calculations from commonly used models. The multiplicity distribution at 7 TeV is described fairly well by the negative binomial distribution.