Underlying event studies with the CMS detector

The ''Underlying Event'' (UE) in a hard scattering process is everything accompanying an event but the hard scattering component of the collision. A CDF analysis showed that the density of particles in the UE of jet events is about a factor of two larger than the density of particles in a typical minimum bias (MB) collision. At the LHC the difference might be even larger. Predictions on the amount of activity in the UE at the LHC based on extrapolations from the lower energy data differ greatly. CMS has demonstrated the feasibility of reference UE measurements under nominal luminosity conditions, assessing our capability to distinguish between the predictions of different models. The UE is studied by examining charged particles in the ''transverse'' region in charged particle jet production and in the central region of Drell-Yan muon-pair production (after removing the muon pair). Further insight into MB and UE comes from studies of heavy quarkonia production in MB and UE. Non-relativistic QCD models introduced from Pythia 6.324 with a special prescription inspired by the multiple interactions framework can reproduce the rates and transverse momenta of J/ψ and Υ observed at Tevatron. (orig.)