After the observation of the Higgs boson in 2012 and thanks to the large luminosity collected during the successful LHC Run 2 data-taking, the properties of the Higgs boson can now be studied in more and more details. In particular, the coupling between the Higgs boson and the top quark, the two most massive elementary particles in the Standard Model (SM), is of high phenomenological interest, as it can be very sensitive to deviations from the Standard Model predictions. One of the key channels to study the top-Higgs coupling is the so-called ttH production mode, where the Higgs boson is produced in association with top quarks and which has been recently observed by the ATLAS collaboration [1]. The study of this channel will be the main focus of this thesis.

The ATLAS detector is ideally suited for such studies, with its design optimised to reconstruct and identify most of the decay products of the Standard Model particles produced in rare physics processes involving a Higgs boson, such as the ttH production mode. This thesis will include some work on the optimisation of the algorithms used to identify jets produced in the hadronization of b-quarks for the upgrade of the ATLAS detector planned for the High-Luminosity phase of the LHC. Those algorithms play a major role in all the final states involving b-quarks, produced in the decay of the top quarks and of the Higgs boson for instance.

Thanks to the large branching fraction of the Higgs boson into b-quarks, the ttH(bb) channel offers the advantage of a relatively large signal acceptance in comparison to the other decay channels. It is possible to exploit this in particular to measure some of the Higgs boson kinematics properties, which can be used to constrain for instance the CP structure of the top-Higgs coupling. Those constraints may shed some light on one of the current biggest unanswered questions of physics, namely why our Universe is nowadays dominated by matter instead of antimatter. This thesis will focus on the detailed study of that channel, benefitting from the very strong expertise of the ATLAS group at CPPM in this area and will involve regular stays at CERN.

Previous experience with C++ programmation and Root data analysis software is required.

[1] Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2018-13/