Measurement of the Underlying Event Activity in the Drell-Yan process in proton-proton collisions at $\sqrt{s

A measurement of the underlying event activity is performed, in proton-proton collisions at a centre-of-mass energy $\sqrt{s}$ = 7 TeV, using data collected with the CMS experiment at the LHC, during 2010. The Drell-Yan process with muonic final state, $q\bar q \to \mu^+\mu^-$, provides an excellent way to study the underlying event activity by separating the hard interaction from the soft component. The observables which are sensitive to the underlying event, the average charged particle density and the average density of the scalar sum of the transverse momentum of the charged particles, have been studied in the directions opposite, along, and transverse to the resultant direction of the two muons, defined in the plane transverse to the beam direction. The underlying event activity is observed to be independent of the di-muon invariant mass in a region between 60 and 120 GeV/c$^2$, implying a saturation of the multiple parton interactions at these energy scales. A slow growth of the underlying event activity is observed with increasing transverse momentum of the di-muon system. The data are corrected to the particle level and compared with the predictions of various non-perturbative models of soft interaction.