Measurement of the boosted WW+WZ production cross section in the semileptonic decay channel with ATLAS

This thesis presents the first measurement of the combined WW+WZ production cross section in the boosted regime with one W boson decaying via the electron or muon channel and the remaining W or Z boson decaying to hadrons. The event selection targets events where the hadronic decay products are highly collimated as a result of a large transverse momentum of the decaying boson. The data that were used consist of the entire dataset of proton-proton collisions at a center-of-mass energy of √(s)=8 TeV collected with the ATLAS detector at the Large Hadron Collider during the 2012 data taking period. The integrated luminosity of this dataset corresponds to 20.3 fb^-1. The main background processes to the WW+WZ signal are W+jets, Z+jets, top quarks and QCD multijet production. The latter is estimated using a dedicated data-driven technique while the other backgrounds are modeled by Monte Carlo simulation. In total 383 signal events are observed in data with an overall estimated background of 12452 events. The corresponding measured fiducial cross section yields: σ^meas_fid(WW+WZ)=(31.7±10.4(stat.)±22.1(syst.)) fb. This result is compatible within the errors with the Standard Model next-to-leading-order prediction of σ^SM_WW+WZ=(57.7±2.9) fb. The observed transverse momentum spectrum of the hadronically decaying boson is used to search for new physics via anomalous triple gauge boson couplings at WWZ and WWγ vertices. The observations are consistent with the Standard Model and confidence intervals are calculated for the parameters describing the anomalous couplings.