Studies of the properties of the Higgs Boson

The Standard Model of particle physics is a very successful theory, describing the fundamental constituents of matter and their interactions. In the past 40 years it has proven to be extremely accurate in predicting the results observed by our experiments, including the presence of the Higgs boson, discovered in 2012 by the ATLAS and CMS collaborations. Despite the great predictive power of the Standard Model, the prevailing theory of particle physics, there are phenomena which are not described by the theory, hinting that new physics must exist beyond the Standard Model, two examples are dark matter and dark energy.

The thesis will report the results of performing precision measurements of the properties of the Higgs boson, in order to look for new physics beyond the Standard Model. Two examples are the mass and the width of the Higgs that provide a powerful test of the theory and their measurement are the principal goals of the thesis. Given the large dataset that ATLAS will collect in the next three years, the H->ZZ->4l channel will provide the most precise measurement of the Higgs mass, especially for the muon final states. This measurement, together with those of other Standard Model parameters such as the W boson and top quark masses, can be used to directly test the consistency of the Standard Model.

Measuring the Higgs boson width provides extremely interesting information as it is sensitive to the decays of the Higgs boson to invisible particles. At the LHC this measurement is not directly possible, given that the expected width is about 4 MeV, almost three orders of magnitude smaller than the experimental resolution. A constraint on the width of the Higgs boson can, however, be extracted from the measurement of the "on-shell" and "off-shell" signal strengths in the H->ZZ->4l (2l2v) final states, if we assume that the couplings of the Higgs boson do not depend on the energy at which it is produced. The (2l2v) final state is also sensitive to new Higgs particles at higher mass that are predicted in theories that extend the Standard Model. A search for these new Higgs particles will also be made if sufficient time exists and the searches continue to be of interest experimentally and theoretically.

In addition to the analysis of the proton-proton collision data, an ATLAS student must make a technical contribution to the experiment in order to be qualified for authorship. This is important for the education of the student and necessary for the experiment in order for it to remain operational and well-monitored. ATLAS is embarking on an ambitious upgrade of the detector and Ricardo will be involved in this through high statistics studies to understand the response of the 130 nm readout electronic ASICs for the silicon camera upgrade to ionizing radiation.