Tese de mestrado. Engenharia Electrotécnica e de Computadores (Área de especialização de Informática e Automação). Faculdade de Engenharia. Universidade do Porto. 2006

Estágio realizado no CERN, Suiça e orientado pelo Eng.º Ronny Billen e Chris Roderick / Tese de mestrado integrado. Engenharia Informátca e Computação. Faculdade de Engenharia. Universidade do Porto. 2008

The highly celebrated discovery of a new particle with a mass of 125 GeV in proton-proton collisions by the ATLAS and CMS experiments at the CERN Large Hadron Collider in 2012 has been shown to be compatible with the Standard Model description of the Higgs boson. However, in order to fully verify the Standard Model nature of the Higgs boson, most of its properties still remain to be measured. Such measurements include differential cross section measurements, which are shown here for the H→WW decay channel and the coupling of the Higgs boson to bottom quarks, for which a study of future prospects is presented. Differential fiducial cross section measurements of the Higgs boson were performed in the H → WW*→lvlv channel at the ATLAS detector with 20 fb−1 of √s = 8 TeV collision data. For Higgs bosons produced by gluon-gluon fusion, the cross section is measured as a function of kinematic variables, including transverse momentum and rapidity of the Higgs boson, as well as the number of jets associated with the Higgs event. The obtained distributions are unfolded to a fiducial volume using a two-dimensional iterative Bayesian algorithm. The measured fiducial differential cross sections are compared to predictions from Monte Carlo generators. The total cross section measured in the fiducial volume defined by the charged lepton and neutrino kinematic properties is 36.0 ± 9.7 fb. Additionally the jet-veto efficiency in the fiducial volume is extracted from the differential cross sections. An analysis is presented of Higgs boson production and decay into bottom quarks in association with a vector boson at the ATLAS detector for the future high-luminosity LHC with proton-proton collisions at √s = 14 TeV. The vector bosons are reconstructed from Z→l+l− or W→lv final states, where l is an electron or muon. The analysis uses generator-level Monte Carlo samples to which efficiency and resolution smearing functions are applied. These reproduce the expected resolution of the upgraded ATLAS detector for the foreseen amount of pile-up due to multiple overlapping proton-proton collisions. The analysis of the ZH(→ l+l−b¯b) channel is presented and results are combined with the WH(→lvb¯b) channel from a corresponding study. For an integrated luminosity of 300 fb−1 using an average pile-up of 60, the expected significance is 3.9 σ with an expected error on the signal strength of 25%. Likewise, for 3000 fb−1 using an average pileup of 140 the expected significance is 8.8 σ , and the error on the signal strength is expected to be about 15%.

A search for heavy exotic diboson resonances decaying to llqq final states is presented using pp collision data collected with the ATLAS detector at the Large Hadron Collider. The analysis uses a data sample corresponding to an integrated luminosity of 20.3 fb^-1 at sqrt(s) = 8 TeV collected between April and December 2012. No significant excess of data events over the predicted Standard Model background is observed and 95% confidence level upper limits are set on the product of the production cross-section and the branching ratio for spin-2 Kaluza-Klein gravitons predicted by the bulk Randall-Sundrum model and for Extended Gauge Model W’ bosons. These results are subsequently combined with limits obtained from searches using the lvll, lvqq, and qqqq final states, and new mass limits are set on both signal models.

With the start of the Large Hadron Collider(LHC) at CERN, we will obtain a new understanding of the physics beyond our current limits. New discoveries will be made; but we will require a deeper understanding, which the LHC machine, being a hadron collider, will not be able to elucidate. Instead, we will need an e+e- collider to make precision measurements of the newly discovered phenomena. Electroweak symmetry breaking and the origin of fermion and boson masses are fundamental issues in our understanding of particle physics. The essential piece of electroweak symmetry breaking - the Higgs boson - will probably be discovered at the LHC. If there are one, or more, Higgs boson(s) precise measurements of all properties of the Higgs will be very important. In this thesis I present two measurements of Standard Model Higgs boson properties in the context of the International Linear Collider (ILC) at √s = 500 GeV, using the proposed International Linear Detector (ILD). First a performance study of ILD to measure the branching ratios of the Higgs boson with mH = 120 GeV, where the Higgs boson is produced with a Z-boson via the Higgsstralung process, and the Z decays into e+e- or μ+μ-. It will also be essential to study the Higgs Yukawa coupling. Therefore, in the second part of this thesis, I present a study of e+e- → tt¯H with the aim of making a direct measurement of the the top-Higgs coupling, using the semi-leptonic nal state and mH of 120 GeV. I show that the top-Higgs coupling can be measured with an accuracy of better than 28%.

Estágio realizado no CERN e orientado Eva Sanchez-Corral Mena / Tese de mestrado integrado. Engenharia Informática e Computação. Faculdade de Engenharia. Universidade do Porto. 2008

Estágio realizado na ATLAS Distributed Computing Group e orientado por Markus Elsing / Tese de mestrado integrado. Engenharia Informátca e Computação. Faculdade de Engenharia. Universidade do Porto. 2008