The pseudorapidity density and multiplicity distribution of charged particles produced in proton-proton collisions at the LHC, at a centre-of-mass energy √s= 7 TeV, were measured in the central pseudorapidity region {pipe}η{pipe} < 1. Comparisons are made with previous measurements at √s = 0.9 TeV and 2.36 TeV. At √s = 7 TeV, for events with at least one charged particle in {pipe}η{pipe} < 1, we obtain dNch/dη = 6.01 ± 0.01(stat.)+0.20-0.12(syst.). This corresponds to an increase of 57.6% ± 0.4%(stat.)+3.6-1.8%(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. © 2010 CERN for the benefit of the ALICE collaboration.

On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider (LHC), two counter-rotating proton bunches were circulated for the first time concurrently in the machine, at the LHC injection energy of 450 GeV per beam. Although the proton intensity was very low, with only one pilot bunch per beam, and no systematic attempt was made to optimize the collision optics, all LHC experiments reported a number of collision candidates. In the ALICE experiment, the collision region was centred very well in both the longitudinal and transverse directions and 284 events were recorded in coincidence with the two passing proton bunches. The events were immediately reconstructed and analyzed both online and offline. We have used these events to measure the pseudorapidity density of charged primary particles in the central region. In the range |η|<0.5, we obtain dNch/dη=3. 10±0. 13(stat.)±0. 22(syst.) for all inelastic interactions, and dNch/dη=3.51±0. 15(stat.)±0. 25(syst.) for non-single diffractive interactions. These results are consistent with previous measurements in proton-antiproton interactions at the same centre-of-mass energy at the CERN Spp̄S collider. They also illustrate the excellent functioning and rapid progress of the LHC accelerator, and of both the hardware and software of the ALICE experiment, in this early start-up phase. © CERN 2009.

Charged-particle production was studied in proton-proton collisions collected at the LHC with the ALICE detector at centre-of-mass energies 0.9 TeV and 2.36 TeV in the pseudorapidity range {pipe}η{pipe}<1.4. In the central region ({pipe}η{pipe}<0.5), at 0.9 TeV, we measure charged-particle pseudorapidity density dNch/dη= 3.02 ± 0.01 (stat.)-0.05+0.08(syst.) for inelastic interactions, and dNch/dη = 3.58 ± 0.01 (stat.)-0.12+0.12(syst.) for non-single-diffractive interactions. At 2.36 TeV, we find dNch/dη = 3.77 ± 0.01 (stat.)-0.12+0.25(syst.) for inelastic, and dNch/dη = 4.43 ± 0.01(stat.)-0.12+0.17(syst.) for non-single-diffractive collisions. The relative increase in charged-particle multiplicity from the lower to higher energy is 24.7% ± 0.5%(stat.)-2.8+5.7(syst.) for inelastic and 23.7% ± 0.5%(stat.)-1.1+4.6% 1.1%(syst.) for non-single-diffractive interactions. This increase is consistent with that reported by the CMS collaboration for non-single-diffractive events and larger than that found by a number of commonly used models. The multiplicity distribution was measured in different pseudorapidity intervals and studied in terms of KNO variables at both energies. The results are compared to proton-antiproton data and to model predictions. © 2010 CERN for the benefit of the ALICE collaboration.