Charged-particle multiplicities in<i>pp</i>interactions measured with the ATLAS detector at the LHC

TLDR

The study measures charged‑particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the correlation between mean transverse momentum and multiplicity. ATLAS recorded proton–proton collisions at 0.9, 2.36, and 7 TeV with a single‑arm minimum‑bias trigger, and the resulting charged‑particle distributions were corrected to well‑defined phase‑space regions using model‑independent methods. The measured multiplicities, both inclusive and diffraction‑reduced, exceed predictions from various Monte‑Carlo models, with central charged‑particle densities of 3.483 ± 0.009 (stat) ± 0.106 (syst) at 0.9 TeV and 5.630 ± 0.003 (stat) ± 0.169 (syst) at 7 TeV.

Abstract

Measurements are presented from proton–proton collisions at centre-of-mass energies of , 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 (MC) models, including a new AMBT1 pythia6 tune. In all the kinematic regions considered, the particle multiplicities are higher than predicted by the MC 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 and 5.630±0.003 (stat)±0.169 (syst) at .