We present a search for the standard model Higgs boson produced with a Z boson in 4.1 fb−1 of data collected with the CDF II detector at the Tevatron. In events consistent with the decay of the Higgs boson to a bottom-quark pair and the Z boson to electrons or muons, we set 95% credibility level upper limits on the ZH production cross section times the H --> b{bar b} branching ratio. Improved analysis methods enhance signal sensitivity by 20% relative to previous searches beyond the gain due to the larger data sample. At a Higgs boson mass of 115 GeV/c2 we set a limit of 5.9 times the standard model value.

In this Thesis a search for Higgs boson production has been performed analysing data gathered by the Collider Detector at Fermilab (CDF) from proton-antiproton (p$\bar{p}$) collisions at the center of mass energy √s = 1.96 TeV. Event selection has been optimized on Higgs decaying into W+W- bosons in order to reconstruct final states with two charged leptons formed by either an electron and a tau or a muon and a tau focusing on taus decaying into hadrons. No evidence of Higgs production has been found analysing 4.8 fb-1 of data and an up- per limit has been set on the production cross section as a function of the mass mH in the range from 130 to 200 GeV/c2 with a 95% Confidence Level. For a Higgs mass hypothesis of 160 GeV/c2 the observed limit is 33.5 times the Standard Model predicted cross section [sigma]SM while the expected upper limit for the same mass is 19.6×[sigma]SM. The high recorded luminosity along with the development of new analysis techniques are expected to improve the CDF sensitivity to the search for Higgs boson with hadronic taus reconstructed in the final state, in the mass range 130 to 200 GeV/c2.

In this work, the interaction between the Higgs boson and the top quark is studied with the proton-proton collisions at 13 TeV provided by the LHC at the CMS detector at CERN (Geneva). At the LHC, these particles are produced simultaneously via the associate production of the Higgs boson with one top quark (tH process) or two top quarks (ttH process). Compared to many other possible outcomes of the proton-proton interactions, these processes are very rare, as the top quark and the Higgs boson are the heaviest elementary particles known. Hence, identifying them constitutes a significant experimental challenge. A high particle selection efficiency in the CMS detector is therefore crucial. At the core of this selection stands the Level-1 (L1) trigger system, a system that filters collision events to retain only those with potential interest for physics analysis. The selection of hadronically decaying τ leptons, expected from the Higgs boson decays, is especially demanding due to the large background arising from the QCD interactions. The first part of this thesis presents the optimization of the L1 τ algorithm in Run 2 (2016-2018) and Run 3 (2022-2024) of the LHC. It includes the development of a novel trigger concept for the High-Luminosity LHC, foreseen to start in 2027 and to deliver 5 times the current instantaneous luminosity. To this end, sophisticated algorithms based on machine learning approaches are used, facilitated by the increasingly modern technology and powerful computation of the trigger system. The second part of the work presents the search of the tH and ttH processes with the subsequent decays of the Higgs boson to pairs of τ lepton, W bosons or Z bosons, making use of the data recorded during Run 2. The presence of multiple particles in the final state, along with the low cross section of the processes, makes the search an ideal use case for multivariant discriminants that enhance the selectivity of the signals and reject the overwhelming background contributions. The discriminants presented are built using state-of-the-art machine learning techniques, able to capture the correlations amongst the processes involved, as well as the so-called Matrix Element Method (MEM), which combines the theoretical description of the processes with the detector resolution effects. The level of sophistication of the methods used, along with the unprecedented amount of collision data analyzed, result in the most stringent measurements of the tH and ttH cross sections up to date.

Abstract: We present a search for standard model Higgs boson production in association with a W boson in proton-antiproton collisions at a center of mass energy of 1.96 TeV. WH candidate events have a signature of a single lepton, missing transverse energy, and two jets. The search looks for candidate events in approximately 2.7 fb−1 of data recorded with the CDF II detector. The high transverse momentum (high-pT) lepton (electron or muon) in the events provides a distinct signature for triggering and most of the events in the dataset come from high-pT lepton triggers. Our analysis improves on prior searches by including events recorded on the Missing Transverse Energy (MET) plus 2 jets trigger with a lepton reconstructed as an isolated high-pT charged particle. We increase the sample purity by identifying ("tagging") long-lived b-hadrons in jets. A neural network combines distinguishing kinematic information into a function optimized for WH sensitivity. The neural network output distributions are consistent with the standard model background expectations and we set limits upper limits on the rate of Higgs production. We set 95% confidence level upper limits on the WH production cross section times branching ratio for Higgs masses from 100 to 150 GeV/c2 and express our results as a ratio of the experimental limit to the theoretical Standard Model production rate. Our limits range from 3.6 (4.3 expected) to 61.1 (43.2 expected) for Higgs masses from 100 to 150 GeV/c2, respectively.