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First Measurement of the Underlying Event Activity at the LHC with √ s = 0.9 TeV

A measurement of the underlying activity in scattering processes with p T scale in the GeV region is performed in proton-proton collisions at √ s = 0.9 TeV, using data collected by the CMS experiment at the LHC. Charged hadron 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 hadrons with pseudorapidity | η | < 2, p T > 0.5 GeV/ c , and azimuthal direction transverse to that of the leading object. Submitted to the European Physical Journal C The