Constraining Standard Model and Beyond Standard Model Higgs Boson Couplings in the Four Lepton Decay Channel with the ATLAS Detector

In 2012, the Standard Model of Particle Physics was "completed" with the discovery of a new particle consistent with a SM Higgs boson. While this discovery marks a monumental achievement for particle physics, there are several gaps in our understanding of physics which the SM cannot explain. Therefore, we necessarily enter a phase beyond discovery where the properties of the Higgs boson must be understood with increasing detail, including deviations from its expected charge-parity (CP) even nature, and couplings to other particles. Higgs boson decays to four leptons (H->4l) provide an ideal avenue for property measurements, due to their high signal-to-background ratio, and the high precision of electron and muon measurements in ATLAS.This thesis presents an analysis which selects H->4l decays in order to measure the signal rates of different Higgs boson production modes. For these measurements, the full 2015+2016 ATLAS data set is used, corresponding to 36.1fb-1 of pp collisions at a centre-of-mass energy of sqrt(s) = 13 TeV. Multivariate techniques are used to isolate the rare vector boson fusion (VBF) production mechanism, allowing for a more precise rate measurement for the process. The measured VBF signal rate is found to be mu_VBF = 3.95^{+1.73}_{-1.48}, which is consistent with the SM prediction of one. Limits are set on the presence and strength of CP-odd and non-SM CP-even Higgs boson couplings to gluons and weak bosons. For each coupling hypothesis tested, the observed limits are compatible with the SM, although the best-fit configurations favour the presence of anomalous couplings.A new ATLAS electron and photon reconstruction algorithm is presented which uses dynamically-sized clusters to recover radiative energy losses by electrons and positrons within the detector. The resultant "superclusters" improve the expected energy resolution of electrons and photon conversions by up to 40%. Expected resolution improvements are also found for a number of topologies relevant to the ATLAS physics programme, with 5-8% improvements in mass resolution found for Z, J/psi->e+e- decays, and up to 5% improvement in H->4l decays.