Charged-particle multiplicities in pp interactions at sqrt(s) = 900 GeV measured with the ATLAS detector at the LHC

TL;DR

This paper reports the first measurements of charged-particle multiplicities in proton-proton collisions at 900 GeV using the ATLAS detector, providing data to test and improve collision models.

Contribution

It presents the initial measurements of charged-particle multiplicities at 900 GeV with the ATLAS detector, comparing results to Monte Carlo models and previous experiments.

Findings

Charged-particle multiplicity per event at eta=0 is 1.333.

Measured multiplicities are 5-15% higher than model predictions.

Data helps refine models of proton-proton collisions.

Abstract

The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range |eta|<2.5 and pT>500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at eta = 0 is measured to be 1.333 +/- 0.003 (stat.) +/- 0.040 (syst.), which is 5-15% higher than the Monte Carlo models predict.