First measurement of the underlying event activity at the LHC with $\sqrt{s} = 0.9$ TeV

A measurement of the underlying activity in scattering processes with pT scale in the GeV region is performed in proton–proton collisions at $\sqrt{s} = 0.9$ TeV, using data collected by the CMS experiment at the LHC. Charged particle production is studied with reference to the direction of a leading object, either a charged particle 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, pT>0.5 GeV/c, and azimuthal direction transverse to that of the leading object.