Study of the Underlying Event in pp collisions with the ALICE detector at the LHC

The bulk of particles produced in a high-energy hadronic collision originates from low-momentum-transfer processes, which are not amenable to a perturbative treatment and need to be modelled phenomenologically. In this thesis we present a measurement of the bulk event-activity or Underlying Event(UE) in pp collisions at ps = 0.9 and 7TeV at the LHC as a function of the hard scale. Di erent regions are de ned with respect to the azimuthal direction of the leading (highest pT) track: Toward, Transverse and Away. The Toward and Away regions collect the fragmentation products of the hardest interaction. The Transverse region is most sensitive to the UE. The study is performed with charged particles above three di erent pT thresholds: 0.15, 0.5 and 1.0 GeV=c. We observe that for values of the leading-track pT above 3{4 GeV/c the bulk particle production becomes independent of the hard scale. In the Transverse region the multiplicity increases by a factor 2{3 between the lower and higher collision energies, depending on the pT threshold considered. Data are compared to PYTHIA 6.4, PYTHIA 8.1 and PHOJET. On average, all models underestimate the UE activity by about 10{30%.