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.

Since the discovery of the Higgs boson in 2012, the diphoton ([gamma][gamma]) decay channel of the Higgs boson has been one of the most potent channels due to the contrast between the smoothly falling background and sharply peaked signal in the diphoton invariant mass. In 2018, with up to 80 fb-1 of data at [square root]s = 13 TeV, ATLAS observed Higgs boson production in association with a pair of top quarks (ttH) with a significance of 5.8 [sigma] by combining measurements in the [gamma][gamma], bb, ZZ, and multi-lepton Higgs decay channels. The ttH production cross-section was measured to be 670 [plus or minus] 90 (stat) +110 -100 (syst) fb. The diphoton channel was one of the main contributors to this result, alone providing a significance of 4.1 [sigma]. With 140 fb-1, a search for non-resonant Higgs pair production in the bb[gamma][gamma] final state was performed. No significant signal was observed and upper limits at 95% confidence level were set. The observed limit on the SM cross-section was 130 fb, or 4.2 times the predicted value. The observed Higgs trilinear coupling modifier was constrained to be between [-1.5, 6.7]. Both the ttH (H [rightwards arrow] [gamma][gamma]) and HH bb[gamma][gamma] analyses will benefit tremendously from the increased statistics expected from the High-Luminosity LHC (HL-LHC). To ensure the continued efficiency of the detector in the harsh HL-LHC environment, ATLAS will install a new Inner Tracker (ITk) consisting of silicon pixel sensors in its innermost layer. At SLAC National Accelerator Laboratory, a variety of electrical tests are performed for the construction of a prototype integrated pixel system, in order to provide early feedback and validation of the ITk design.

The work presented in this PhD dissertation is the first search at CMS for Higgs bosons produced in association with top quarks (ttH) in a final state consisting of only jets. The results presented in this book uncover a new class of ttH events that will help us elucidate our understanding of the Yukawa sector interactions between the Higgs boson and the top quark. Despite this being the most common decay signature for ttH, a large contamination of SM backgrounds makes it the most challenging for extracting a signal from data. The PhD thesis presents many sophisticated tools and techniques that were developed in order to overcome these challenges. These tools pave the way for future analyses to investigate other standard model and beyond-standard model physics.

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.

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.

Precision measurements of the Higgs boson’s properties are a powerful tool to look for deviations from the predictions of the Standard Model (SM) of particle physics. The 139/fb of proton-proton collision data which have been collected by the ATLAS experiment during Run 2 of the LHC, offer an opportunity to investigate rare Higgs-boson topologies, which are particularly sensitive to new physics scenarios but experimentally difficult to access. Several such measurements, which target Higgs-boson decays to heavy-flavour quarks, as well as their combinations are presented in this thesis. A novel analysis that measures Higgs-boson production in association with a heavy vector boson V (VH, with V=W,Z) at high energies is presented. Dedicated Higgs-boson reconstruction techniques are applied to reconstruct the highly Lorentz-boosted Higgs-boson decays into pairs of bottom quarks. The measurement is subsequently combined with a VH cross-section measurement at low and intermediate pT(V) to provide a differential cross-section measurement in kinematic fiducial volumes over the largest possible pT(V) range. All cross-section measurements agree with the SM predictions within relative uncertainties that range from 30% to 300%. The results are furthermore interpreted as limits on the parameters of a SM effective field theory. Finally, a combination of measurements of Higgs decays to heavy-flavour quarks is used to experimentally determine that the Higgs-boson coupling to charm quarks is weaker than to bottom quarks, as predicted by the SM. The target audience for the thesis are physicists and physics students, in particular those with a background in high energy physics.

This eBook is a collection of articles from a Frontiers Research Topic. Frontiers Research Topics are very popular trademarks of the Frontiers Journals Series: they are collections of at least ten articles, all centered on a particular subject. With their unique mix of varied contributions from Original Research to Review Articles, Frontiers Research Topics unify the most influential researchers, the latest key findings and historical advances in a hot research area! Find out more on how to host your own Frontiers Research Topic or contribute to one as an author by contacting the Frontiers Editorial Office: frontiersin.org/about/contact.