Search for flavour-changing neutral current couplings between the top quark and the Higgs boson in multilepton final states with the ATLAS detector

These proceedings present a search for flavour-changing neutral-current (FCNC) interaction involving the top quark, Higgs boson and either the up or the charm quark, using 140 fb${}^{-1}$ of 13 TeV proton–proton collision data from the ATLAS detector at the Large Hadron Collider. Two channels are considered: the production of top quark-antiquark pair with one top decaying via FCNC, and the associated production of a top quark and Higgs boson. Final states contain either two same-charge leptons, or three leptons of which two have the same charge. Observed (expected) upper limits on the branching rations are determined as $\mathcal{B}(t\to Hu)<2.8(3.0)\times {10}^{-4}$ and $\mathcal{B}(t\to Hc)<3.3(3.8)\times {10}^{-4}$.