Measurement of W+W- Production in pp Collisions at s = 8 TeV and Probing Anomalous Triple-Gauge-Boson Couplings with the ATLAS Detector.

Abstract

This thesis presents the measurement of the vector boson pair W^+W^- production cross section in proton-proton collisions at the center-of-mass energy sqrt(s) = 8 TeV. The leptonic decay channels of the WW+ll for l=(e,mu) are analyzed using data corresponding to 20.3 fb^-1 of integrated luminosity collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN (in Geneva, Switzerland). The experimental signature of this measurement is two energetic isolated leptons (e^+e^-, mu^+mu^-, e^+mu-, e^-mu^+) and associated large missing transverse energy (due to neutrinos in final states). A total of 6636 WW+ll candidate events is selected in ATLAS data with an estimation of 1547+/-28 background events from non-W^+W^- production processes. The measured total production cross section is 71^(+1.1)_(-1.1)(stat)^(+5.7)_(-5.0)(syst)^(+2.1)_(-2.0)(lumi) pb,, which is comparable with the theoretical prediction of 63.2^(+2.0)_(-1.8) pb calculated with NNLO QCD and NLO EW corrections. The anomalous triple-gauge-boson couplings (WWZ and WWgamma) could signal new physics beyond the Standard Model at much higher energy scales compared to the directly detectable mass scale at the LHC. An effective Lagrangian is used to generalize the anomalous triple-gauge-boson couplings to describe the W^+W^- productions at the LHC. These anomalous couplings can be experimentally probed by comparing the leading lepton transverse momentum spectrum with the theoretical predictions in different triple-gauge-boson coupling space. No observation of deviations from the Standard Model predicted couplings is found by a maximum likelihood fitting of the leading lepton transverse momentum. Therefore, the most stringent limits to date on the anomalous triple-gauge-boson couplings are set from this analysis.