Track-based underlying event measurements in pp collisions at sqrt{s} = 900 GeV and 7 TeV with the ATLAS Detector at the LHC

The first underlying event measurements with the ATLAS detector at the LHC are presented. Data were collected using a minimum-bias trigger in December 2009 and March-April 2010 during proton-proton collisions at center of mass energies of 900 GeV and 7 TeV respectively. The observables sensitive to the underlying event, i.e the charged particle density and the sum and average charged transverse momentum are measured as a function of the leading track transverse momentum. The angular distribution of the charged tracks and their transverse momentum with respect to the leading track is also presented, as well as the correlation between mean transverse momentum and charged particle multiplicity. The data show a higher underlying event activity than that predicted by the Monte Carlo models to which it is compared.