Underlying events and jet reconstruction in CMS

In the following, we report on the techniques adopted by the CMS experiment to measure jets and on studies of the underlying event in p p interactions at √s = 0.9 and 7 TeV. Jets are reconstructed in the CMS detector on basis of the energy measured in the calorimeters (Calorimeter jet) or tracks from charged particles (Track-Jet), or by combining both information (Jet-Plus-Track and Particle Flow algorithms). Results from detailed Monte Carlo studies are compared with the first p p data collected at the Lhc and the jet energy response and resolutions are measured leading to a 10% jet energy scale uncertainty for Calorimeter jets (Jet-Plus-Track and Particle Flow jets), with an additional 2% uncertainly per unit of rapidity. The underlying event is studied by measuring the charged particle multiplicity and the energy density in the region perpendicular to the plane of the hard 2-to-2 scattering which includes the beam and the jet directions.The underlying event activity increase with the leading jet transverse momentum (pT,jet) up to about 5-10 GeV/c and a plateau is observed at higher pT,jet values. A factor two increase of the underlying event activity is observed at √s = 7 TeV with respect to 0.09 TeV. These studies allow to discriminate several QCD Monte Carlo parametrization which reproduce the Tevatron underlying even observations but diverge at higher energy as well as improve these models.