The Underlying Event in pp Collisions at 900 GeV

The underlying event in pp interactions at sqrt(s) = 900 GeV is studied measuring the charged multiplicity density and the charged energy density in a region perpendicular to the plane of the hard 2-to-2 scattering. Two different methodologies are adopted to identify the direction and the energy scale of the hard scattering in Minimum Bias events that rely on the leading charged track and on the leading charged jet. The study allows to discriminate between various QCD Monte Carlo models with different multiple parton interaction schemes which correctly reproduce Tevatron and RHIC Underlying Event data but give different predictions when extrapolated to different energies.