Probing the CP nature of the Higgs coupling to top quarks with the ATLAS experiment at the LHC

Since the Higgs boson with a mass of 125 GeV was discovered by the ATLAS and CMS experiments at CERN, the study of the properties of this particle has been a priority of the Large Hadron Collider (LHC) physics programme. The interactions of the Higgs boson with other elementary particles are predicted by the Standard Model of particle physics and can be measured with unprecedented precision using data from the Run 2 of the LHC. The Yukawa couplings between the Higgs boson and fermions may suffer observable effects if the Higgs sector is extended to include additional interactions which violate the symmetry under CP (charge conjugation and parity), an ingredient required to explain the observed imbalance between matter and anti-matter in the Universe. The strongest Yukawa coupling is that of the top quark, which makes it more easily accessible experimentally and also grants it a special role in theoretical issues, such as the naturalness of the Higgs boson mass and the stability of the vacuum. Currently, the best direct probe to measure the top quark Yukawa coupling is the production, at the LHC, of Higgs bosons in association with top quark pairs ($t\bar{t}H$). In this thesis, two analyses are presented of $t\bar{t}H$ events in final states with leptons and in which the Higgs boson decays to a pair of bottom quarks. Data collected by the ATLAS experiment during the Run 2 of the LHC is used, making up a total integrated luminosity of 139 fb$^{−1}$ of proton-proton collisions with a centre-of-mass energy of 13 TeV. The first analysis is a measurement of the $t\bar{t}H$ production cross-section, resulting in the observed signal strength 0.43$^{+0.36}_{−0.33}$. The second analysis is a measurement of the CP structure of the top quark Yukawa coupling, in which the observed CP mixing angle is α = ${4^\circ}^{+52^\circ}_{-60^\circ}$ . Both results are compatible with the Standard Model predictions, although the observed signal rate in this particular final state is not enough to establish evidence for $t\bar{t}H$ production nor to exclude a pure CP-odd coupling with a 95% confidence level.