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 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 B(t→ Hu)<2.8 (3.0) × 10⁻⁴ ℬ ( t → H u ) < 2.8 ( 3.0 ) × 10 − 4 and B(t→ Hc)<3.3 (3.8) × 10⁻⁴ ℬ ( t → H c ) < 3.3 ( 3.8 ) × 10 − 4 .