Measurement of the Underlying Event Activity in Proton-Proton Collisions at 900 GeV

First measurement of the underlying event activity is performed in proton-proton collisions at the LHC with $\sqrt{s} = 900\gev$. The interaction scale is provided by a leading charged particle or jet with transverse momentum larger than $1-3~\gevc$. Charged hadron production is studied in the region transverse to the direction of the leading object. Predictions of several QCD models, after full detector simulation, are compared to the uncorrected data. They predict generally too low hadronic activity in the transverse region. Compared to previous measurements, a strong energy dependence of the multiple parton interaction activity is required to describe the data.