Charged-particle multiplicities in pp interactions measured with the ATLAS detector at the LHC

TL;DR

This paper reports measurements of charged-particle multiplicities in proton-proton collisions at various energies using the ATLAS detector, revealing higher multiplicities than predicted by existing Monte Carlo models.

Contribution

It provides detailed, model-independent measurements of charged-particle multiplicities at multiple energies, including diffraction-reduced and inclusive regions, and compares them with Monte Carlo predictions.

Findings

Charged-particle multiplicities are higher than model predictions.

Measured multiplicity at 0.9 TeV is 3.483, at 7 TeV is 5.630.

Results include new data on the relationship between transverse momentum and multiplicity.

Abstract

Measurements are presented from proton-proton collisions at centre-of-mass energies of sqrt(s) = 0.9, 2.36 and 7 TeV recorded with the ATLAS detector at the LHC. Events were collected using a single-arm minimum-bias trigger. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the relationship between the mean transverse momentum and charged-particle multiplicity are measured. Measurements in different regions of phase-space are shown, providing diffraction-reduced measurements as well as more inclusive ones. The observed distributions are corrected to well-defined phase-space regions, using model-independent corrections. The results are compared to each other and to various Monte Carlo models, including a new AMBT1 PYTHIA 6 tune. In all the kinematic regions considered, the particle multiplicities are higher than predicted by the Monte Carlo models. The central charged-particle multiplicity per event and unit of pseudorapidity, for tracks with pT >100 MeV, is measured to be 3.483 +- 0.009 (stat) +- 0.106 (syst) at sqrt(s) = 0.9 TeV and 5.630 +- 0.003 (stat) +- 0.169 (syst) at sqrt(s) = 7 TeV.