Measurement of the $t\bar{t}H$ Production Cross-Section in the Non-Resonant Four Lepton Final State in $pp$ Collisions at $\sqrt{s}=13\,\text{TeV}$ with the ATLAS Detector

This thesis presents the measurements of the associated production of a top quark pair with a Standard Model Higgs boson in multi-lepton final states, focusing on the measurement of the inclusive cross-section in the $4\ell$ channel. The analysis uses the full Run 2 data set, corresponding to an integrated luminosity of $140\,\text{fb}^{-1}$, collected in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13\,\text{TeV}$. The analysis covers six final states, defined by the number and flavour of charged-lepton candidates. Machine learning techniques are used for all channels to improve signal sensitivity. Leading prompt background contributions from $t\bar{t}Z$, $t\bar{t}W$, and diboson production are constrained by additional regions populated with those backgrounds. Moreover, contributions from fake and non-prompt leptons are estimated using data-driven methods such as the fake factor or template method. The observed signal strength, the ratio of the measured signal yield over the Standard Model prediction, is extracted using a profile likelihood fit. For the $4\ell$ channel, the observed signal strength is $\mu_{t\bar{t}H}=0.53^{+0.67}_{-0.53}=0.53^{+0.66}_{-0.51}\left(\text{stat.}\right)^{+0.13}_{-0.13}\left(\text{sys.}\right)$, corresponding to an observed (expected) significance of $0.95\sigma$ ($1.80\sigma$). The measured signal strength for the combination of all six channels is $\mu_{t\bar{t}H}=0.63^{+0.20}_{-0.19}=0.63^{+0.17}_{-0.16}\left(\text{stat.}\right)^{+0.11}_{-0.10}\left(\text{sys.}\right)$. It corresponds to an observed (expected) significance of $3.3\sigma$ ($5.3\sigma$). The measured inclusive cross-section is found to be $321^{+102}_{-99}\,\text{fb}$. The compatibility is $7.2\,\%$ with the Standard Model prediction of $507^{+35}_{-50}\,\text{fb}$. The signal strength is also measured in bins of the Higgs boson transverse momentum and demonstrates a compatibility of $28\,\%$ with the Standard Model prediction.