  Measurements of charged particle distributions, sensitive to the underlying
event, have been performed with the ATLAS detector at the LHC. 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, pt density, and average pt are measured. The data show a
higher underlying event activity than that predicted by Monte Carlo models
tuned to pre-LHC data.
