STUDIES OF COSMIC RAYS AND MINIMUM BIAS MEASUREMENTS IN PROTON-PROTON COLLISIONS AT (s)^1/2 = 0.9 and 7 TeV WITH THE ATLAS DETECTOR AT THE LHC

Abstract

The first measurement of lower momentum charged particle multiplicities made by the ATLAS experiment at the LHC are presented. The charged particle multiplicities are measured as a function of transverse momentum, pseudorapidity, and charged particle multiplicity. These measurements are made in three different phase-space regions of at least two charged particles in the kinematic range of |&eta;| < 2.5 and pT > 100 MeV, at

least one charged particle in the kinematic range of |&eta;| < 2.5 and pT >500 MeV, and

at least six charged particles in the kinematic range of |&eta;| < 2.5 and pT > 500 MeV. The relation between average transverse momentum and pseudorapidity with charged particle multiplicity are also measured. Two different center-of-mass energies, 0.9 TeV

and 7 TeV, are used to measure the charged particle multiplicities. The measured distributions are compared to predictions from Monte Carlo models. All the Monte Carlo models predict the charged particle multiplicity lower than the measured in all

three phase-space regions. Using these results, the Monte Carlo models are updated, and the updated models predict a better description of properties of charged particles at the LHC.