This thesis presents innovative contributions to the CMS experiment in the new trigger system for the restart of the LHC collisions in Run II, as well as original analysis methods and important results that led to official publications of the Collaboration. The author's novel reconstruction algorithms, deployed on the Field-Programmable Gate Arrays of the new CMS trigger architecture, have brought a gain of over a factor 2 in efficiency for the identification of tau leptons, with a very significant impact on important H boson measurements, such as its decays to tau lepton pairs and the search for H boson pair production. He also describes a novel analysis of HH → bb tautau, a high priority physics topic in a difficult channel. The original strategy, optimisation of event categories, and the control of the background have made the result one of the most sensitive concerning the self-coupling of the Higgs boson among all possible channels at the LHC.

This dissertation presents the first observation of Higgs boson production in association with a top quark-antiquark pair in the diphoton decay channel, with a significance of 6.6 standard deviations. The measurement is performed with a dataset of 13 TeV proton-proton collisions recorded by the Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of 137 fb−1.

During Run 2 of the Large Hadron Collider, the ATLAS experiment recorded proton-proton collision events at 13 TeV, the highest energy ever achieved in a collider. Analysis of this dataset has provided new opportunities for precision measurements of the Higgs boson, including its interaction with the top quark. The Higgs-top coupling can be directly probed through the production of a Higgs boson in association with a top-antitop quark pair (ttH). The Higgs to diphoton decay channel is among the most sensitive for ttH measurements due to the excellent diphoton mass resolution of the ATLAS detector and the clean signature of this decay. Event selection criteria were developed using novel Machine Learning techniques to target ttH events, yielding a precise measurement of the ttH cross section in the diphoton channel and a 6.3 $\sigma$ observation of the ttH process in combination with other decay channels, as well as stringent limits on CP violation in the Higgs-top coupling.

This book provides a comprehensive overview of the field of Higgs boson physics. It offers the first in-depth review of the complete results in connection with the discovery of the Higgs boson at CERN’s Large Hadron Collider and based on the full dataset for the years 2011 to 2012. The fundamental concepts and principles of Higgs physics are introduced and the important searches prior to the advent of the Large Hadron Collider are briefly summarized. Lastly, the discovery and first mensuration of the observed particle in the course of the CMS experiment are discussed in detail and compared to the results obtained in the ATLAS experiment.

Here, we presented a measurement of differential cross sections for the Higgs boson (H) production in pp collisions at √s = 8 TeV. The analysis exploits the H →?? decay in data corresponding to an integrated luminosity of 19.7 fb-1 collected by the CMS experiment at the LHC. The cross section is measured as a function of the kinematic properties of the diphoton system and of the associated jets. Results corrected for detector effects are compared with predictions at next-to-leading order and nextto-next-to-leading order in perturbative quantum chromodynamics, as well as with predictions beyond the standard model. Furthermore, for isolated photons with pseudorapidities.

The recent observation of the Higgs boson has been hailed as the scientific discovery of the century and led to the 2013 Nobel Prize in physics. This book describes the detailed science behind the decades-long search for this elusive particle at the Large Electron Positron Collider at CERN and at the Tevatron at Fermilab and its subsequent discovery and characterization at the Large Hadron Collider at CERN. Written by physicists who played leading roles in this epic search and discovery, this book is an authoritative and pedagogical exposition of the portrait of the Higgs boson that has emerged from a large number of experimental measurements. As the first of its kind, this book should be of interest to graduate students and researchers in particle physics.

This book highlights the most complete characterization of the Higgs boson properties performed to date in the "golden channel," i.e., decay into a pair of Z bosons which subsequently decay into four leptons. The data collected by the CMS experiment in the so-called Run-II data-taking period of the LHC are used to produce an extensive set of results that test in detail the predictions of the Standard Model. Given the remarkable predictive power of the SM when including the Higgs boson, possible new physics will require even more extensive studies at higher statistics. A massive upgrade of the detectors is necessary to maintain the current physics performance in the harsh environment of the High-Luminosity LHC (HL-LHC) project, expected to start by the end of 2027. The CMS Collaboration will replace the current endcap calorimeters with a High Granularity Calorimeter (HGCAL). The HGCAL will be the very first large-scale silicon-based imaging calorimeter ever employed in a high-energy physics experiment. This book presents the results of the analysis of the test beam data collected with the first large-scale prototype of the HGCAL. The results of this analysis are used to corroborate the final design of the HGCAL and its nominal physics performance expected for the HL-LHC operations.

In July 2012, a Higgs boson was discovered by both the ATLAS and CMS experiments. This thesis presents the most recent version of the H ->[gamma][gamma] analysis that was used in the discovery. Since the discovery, focus has shifted to measure the properties of the Higgs boson. The analysis presented here is the analysis of the Higgs couplings, where events are divided into twelve categories defined to isolate the various production processes. Specific focus is given to the vector boson fusion (VBF) process where the Higgs boson is produced in association with two foward/backward jets. Due to its small theory errors, the VBF process can allow for very precise measurements of the Higgs boson's properties. Also presented are the first differential cross section measurements for the Higgs boson as measured in the diphoton decay channel. Distributions in data are unfolded to particle level and compared with the latest Monte Carlo event generators for gluon-gluon fusion. Many distributions concentrate on the kinematics of the Higgs boson produced in association with jets. Finally, a proposal for a 1-jet VBF region for the high-luminosity LHC is proposed based on the difference in rapidity between the Higgs boson and the leading jet. This region shows good discrimination power against gluon-gluon fusion events and has been shown to discriminate against beyond the Standard Model couplings.

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.