Measurement of the Underlying Event Activity at the LHC with $\sqrt{s

A measurement of the underlying activity in scattering processes with a $p_T$ scale in the several GeV region is performed in proton-proton collisions at $\sqrt{s} = 7 \TeV$, using data collected by the CMS experiment at the LHC. Charged particle production is studied with reference to the azimuthal direction of the leading set of charged particles forming a ``track-jet". A significant growth of the average multiplicity and of the average scalar $p_T$ sum of charged particles with a pseudorapidity \etacut, \ptcut, and azimuthal direction transverse to that of the leading track-jet is observed with increasing scale provided by the leading track-jet $p_T$, followed by saturation at large values of the scale. A hardening of the multiplicity distribution, of the scalar $p_T$ sum distribution and of the charged particle $p_T$ spectrum is also observed with increasing scale values. A significant growth of the activity in the transverse region is also observed, for the same value of the leading track-jet $p_T$, from $\sqrt{s} = 0.9 \TeV$ to $\sqrt{s} = 7 \TeV$. Predictions of several QCD-inspired models as implemented in PYTHIA, with full detector simulation, are compared to the data.