The underlying event in proton-proton collisions at 900 GeV from CMS

It is presented a measurement of the underlying activity in scattering processes with pT scale in the GeV region performed in proton-proton collisions at 0.9TeV, using data collected by the CMS experiment at the LHC. or a set of charged particles forming a jet. Predictions of several QCD-inspired models as implemented in PYTHIA are compared, after full detector simulation, to the data. The models generally predict too little production of charged particles with pseudorapidity |η|<2, transverse momentum pT>0.5GeV/c and azimuthal direction transverse to that of the leading object.