Search for flavour-changing neutral current interactions in the top-quark Higgs boson sector in multi-lepton final states with the ATLAS detector at the LHC at $\sqrt{s} = 13\,\text{TeV}$

A search for flavour-changing neutral currents in the top-quark Higgs boson sector is presented, utilizing an integrated luminosity of $\mathcal{L}_{\text{int}} = 140 \,\text{fb}^{-1}$ of proton-proton collisions recorded by the ATLAS experiment at a centre-of-mass energy of $\sqrt{s} = 13\,\text{TeV}$. As flavour-changing neutral currents are highly suppressed in the Standard Model of particle physics, these processes are sensitive to physics beyond the Standard Model. To differentiate between signal and background, advanced reconstruction methods are employed for creating kinematic distributions. Multivariate analysis techniques are utilised to optimize the differentiation between signal and background, leading to expected upper exclusion limits on the branching ratio $\mathcal{B}\left(t\rightarrow qH\right)$ in a binned profile likelihood fit including a full set of systematic uncertainties. The obtained expected exclusion limits amount to $\mathcal{B}\left(t\rightarrow uH\right) < 4.1\cdot 10^{-4}$ and $\mathcal{B}\left(t\rightarrow cH\right) < 5.0\cdot 10^{-4}$ for the branching ratio corresponding to the utH and ctH couplings respectively.