Measurement of the Underlying Event Activity in $pp$ collisions at $\sqrt{s} = 0.9$~TeV and 7 TeV

A measurement of the underlying activity in scattering processes with a $p_T$ scale of several GeV/$c$ is performed in proton-proton collisions at $\sqrt{s} = 7$~TeV using corrected data collected by the CMS experiment at the LHC. The production of charged particles is studied with reference to the azimuthal direction of the leading track-jet with $|\eta| < 2$ and $p_T > 0.5$~GeV/$c$. A significant increase of the average number of charged particles and of the average scalar $p_T$ sum is observed followed by a saturation at large values of the $p_T$ scale. A significant growth of the activity in the transverse region is observed when increasing the leading track-jet cut for the same energy or when increasing the energy from $\sqrt{s} = 0.9$~TeV to $\sqrt{s} = 7$~TeV. Various PYTHIA based QCD-inspired models are compared with the data after full detector simulation.