Measurements of Higgs boson properties in associated production with top quarks with the ATLAS detector

Measurements of Higgs boson production in association with top quarks were performed using 139 fb$^{-1}$ of data collected with the ATLAS detector at the Large Hadron Collider. The analyses target events with one or two electrons or muons in the final state and the $H\rightarrow b\bar{b}$ decay mode. The $t\bar{t}H(H\rightarrow b\bar{b})$ cross-section was measured differentially in bins of Higgs boson transverse momentum. An excess of events over the predicted background hypothesis was observed with a significance of $1.0\sigma$ ($2.7\sigma$ expected). The signal strength was measured to be $0.35\pm0.20\text{(stat.)}^{+0.30}_{−0.28}\text{(syst.)}$. The charge-parity (CP) structure of the top-Higgs Yukawa coupling was measured for the first time in the $H\rightarrow b\bar{b}$ channel and targeting the $t\bar{t}H$ and $tH$ production processes. Novel angular variables based on the reconstructed top quarks transverse momenta were used to discriminate between different CP scenarios. The mixing angle between CP-even and CP-odd couplings is measured to be $11^{\circ~+52^{\circ}}_{-73^{\circ}}$. The analyses sensitivity is limited by systematic uncertainties associated with the modelling of the dominant $t\bar{t}+\geq1b$ background. Measurements of top quark pair production in association with c-jets were performed using 140 fb$^{-1}$ of data. The cross-section of $t\bar{t}+\geq2c$ is found to be $\sim30$ % larger than predicted. Studies related to the development of the ATLAS trigger system were also performed. For Run 3, triggers targeting the central exclusive production of jets were implemented, combining the central and AFP detectors. For the High-Luminosity LHC, the performance of a hardware tracking trigger in signatures with two muons was evaluated using a software emulation.