The Road to Discovery: Detector Alignment, Electron Identification, Particle Misidentification, WW Physics, and the Discovery of the Higgs Boson

The Standard Model of particle physics has been tested by many experiments and describes all observed phenomena up to the highest particle interaction energies. The existence of a scalar particle, the Higgs boson, is central to the theory. The Higgs boson was the only fundamental particle that had not been observed prior to the turn-on of the Large Hadron Collider (LHC). This thesis describes a progression of research that builds to a search for the Higgs boson using the ATLAS detector at the LHC. The search uses the signature of the Higgs boson decaying to a pair of W bosons (WW). Both W bosons are required to decay leptonically into a charged lepton and a neutrino. This signature suffers from many sources of background; the most important are continuum electroweak WW production and the production of single W bosons accompanied by a jet misidentified as a lepton (W+jet background). To understand and quantify these backgrounds, a measurement of the WW cross section has been performed, and analysis techniques have been developed to model the W+jet background. This thesis presents the measurement of the WW cross section using 1.02 $fb^{-1}$ of $sqrt{s} = 7$ TeV collision data and documents the method for modeling the W+jet background. Understanding the detector is a crucial first step in these analyses. Two commissioning activities are described: detector alignment and prompt electron identification. Detector alignment is needed to accurately reconstruct the trajectory of charged particles in the ATLAS Inner Detector (ID). This thesis documents the alignment of the Transition Radiation Tracker, a key component of the ID. Charged leptons (electrons and muons) are signatures of many of the most interesting physics processes at hadron colliders, and the efficient and reliable identification of charged leptons is critical to the physics program at ATLAS. This thesis describes work on electron identification used both for real-time selection of interesting events and for physics analysis. Finally, the search for the Higgs boson in the H $ ightarrow ext{WW} ightarrow l u l u$ channel is presented using 4.7 $fb^{-1}$ of $sqrt{s} = 7$ TeV collision data and 5.8 $fb^{-1}$ of $sqrt{s} = 8$ TeV collision data.