Measurement of underlying event characteristics using charged particles in pp collisions at sqrt{s} =900 GeV and 7 TeV with the ATLAS detector in a limited phase space

Measurements of charged particle distributions, sensitive to the underlying event, is presented in a restricted phase-space to directly compare with the results from the other LHC experiments. The measurements are based on data collected using a minimum-bias trigger to select proton--proton collisions at center-of-mass energies of 900 GeV and 7 TeV. The ``underlying event'' is defined as those aspects of a hadronic interaction attributed not to the hard scattering process, but rather to the accompanying interactions of the rest of the proton. Three regions are defined in azimuthal angle with respect to the highest pT charged particle in the event, such that the region transverse to the dominant momentum-flow is most sensitive to the underlying event. In each of these regions, distributions of the charged particle multiplicity and pT density, as a function of the transverse momentum of the leading charged particle, are measured. Two kinematic ranges are analyzed on which ATLAS, CMS and ALICE agreed, defined by requiring at least one well-defined track with |eta| <0.8 and a minimum pT of 500 MeV or 1 GeV, respectively. The data show a higher underlying event activity than that predicted by Monte Carlo models tuned to pre-LHC data.