Measurement of the Underlying Event Activity at the LHC at 7 TeV and Comparison with 0.9 TeV

A measurement of the underlying event activity is performed on proton-proton collisions for $\sqrt{s} = 0.9$ TeV and $7$ TeV at CMS. The charged particles in the azimuthal region transverse to the leading track with $p_{T}$ $ > 0.5$ GeV/c and within $|\eta|\!<\!0.8$ are studied. The aim of the study is to compare the measurements with various MC predictions to get extra constraints for the tuning parameters, as well as getting a better understanding for the underlying event phenomena. A significant increase in the average multiplicity and the scalar $\sum p_T$ in the transverse region is observed up to a leading track $p_T$ of few GeV/c followed by a slower rate of increase. Both quantities are observed to increase with $\sqrt{s}$.