Charged-particle multiplicity measurement in proton-proton collisions at $\sqrt{s}$ = 7 TeV with ALICE at LHC

TL;DR

This paper reports measurements of charged-particle multiplicities in proton-proton collisions at 7 TeV with ALICE, revealing a significant increase compared to lower energies and challenging existing models.

Contribution

First measurement of pseudorapidity density and multiplicity distribution at 7 TeV with detailed comparison to models.

Findings

Charged-particle density at 7 TeV is 6.01 ± 0.01 (stat.) with systematic uncertainties.

The multiplicity increase from 0.9 TeV is approximately 57.6%.

Multiplicity distribution fits well with a negative binomial distribution.

Abstract

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 |$\eta$| < 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 |$\eta$| < 1, we obtain dNch/deta = 6.01 $\pm$ 0.01 (stat.) $^{+0.20}_{-0.12}$ (syst.). This corresponds to an increase of 57.6% $\pm$ 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.