Underlying event 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 pp interactions at $\sqrt{s} = 0.9$ 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 pp data collected at the LHC and the jet energy response and resolutions are measured leading to a 10\% (5\%) jet energy scale uncertainty for Calorimeter jets (Jet--Plus--Track and Particle Flow jets), with an additional 2\% uncertainty per unit of rapidity. % The underlying event is studied by measuring the charged particle multiplicity and the energy density in the regions 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 ($p_{T,\rm{jet}}$) up to about $5-10$ GeV/c and a plateau is observed at higher $p_{T,\rm{jet}}$ values. A factor two increase of the underlying event activity is observed at $\sqrt{s} = 7$ TeV respecting to $0.9$ TeV. These studies allow to discriminate among several QCD Monte Carlo parametrisations which reproduce the Tevatron underlying event observations but diverge at higher energy as well as improve these models.