From Discovery to Precision Measurements: Electron Identification, Electron-Like Backgrounds, and Measurement of the Differential Fiducial Cross-Sections of the Higgs Boson in the Four-Lepton Decay Channel with the ATLAS Detector

The Higgs boson was proposed in 1964 as part of a solution to the problem of how certain subatomic particles acquire mass. Until its discovery at CERN's Large Hadron Collider in 2012, the Higgs boson was the last missing piece of the Standard Model of particle physics. This thesis studies the Higgs boson in the four-lepton decay channel: H->ZZ*->4l, where the leptons may be electrons or muons, using ~25 inverse femtobarns of data recorded by the ATLAS detector at centre-of-mass energies of 7 and 8 TeV in 2011 and 2012, respectively. Particular attention will be given to the role of electrons in the search for and study of the Higgs boson. An offline electron identification algorithm, the MultiLepton menu, was developed in order to efficiently distinguish true reconstructed electron objects from the hadronic background. It achieved an efficiency of ~95% and excellent background rejection, and contributed to the discovery of the Higgs boson. The reducible electron-like background in the H->ZZ*->4l channel has been studied, and a data-driven method for estimating the background contributions of the Z+jets, Z+bb, and tt processes was developed and implemented as a cross-check for the final analysis, published in 2014. Several precision measurements of the Higgs boson's properties have been performed in the H->ZZ*->4l channel, including the first measurements of the inclusive and differential fiducial cross-sections, using the method of bin-by-bin correction factors. The inclusive cross-section has been measured to be 2.11 +0.53/-0.47(stat.) +/- 0.8(syst.) fb. Differential cross-sections have been measured for six observables; no significant deviations from theoretical Standard Model predictions have been observed. An alternative method for extracting the differential cross-sections, the Bayesian iterative unfolding method, has been studied and has been found to yield results consistent with the nominal results. This method shows particular promise for future precision measurements in the high-statistics regime, where resolution effects may degrade the performance of the method of correction factors.