Search for flavour-changing neutral-current couplings between the top quark and the Higgs boson in multi-lepton final states in 13 TeV pp collisions with the ATLAS detector

 H A search is presented for flavour-changing neutral-current interactions involving the top quark, the Higgs boson and an up-type quark (q = u, c) with the ATLAS detector at the Large Hadron Collider. The analysis considers leptonic decays of the top quark along with Higgs boson decays into two W bosons, two Z bosons or a τ⁺τ^- pair. It focuses on final states containing either two leptons (electrons or muons) of the same charge or three leptons. The considered processes are tt^- and Ht production. For the production, one top quark decays via t → Hq. The proton–proton collision data set analysed amounts to (140 fb^-1)at (√s = 13 TeV). No significant excess beyond Standard Model expectations is observed and upper limits are set on the t → Hq branching ratios at 95 % confidence level, amounting to observed (expected) limits of B( t → Hq)  < 2.8 (3.0) x 10-4 and B (t → Hc) < 3.3 (3.8) x 10-4. Combining this search with other searches for tHq flavour-changing neutral-current interactions previously conducted by ATLAS, considering H → bb^- and H → γγ decays, as well as H → τ⁺τ^- decays with one or two hadronically decaying τ-leptons, yields observed (expected) upper limits on the branching ratios of B ( t→ Hu ) < 2.6 (1.8) x 10^-4 and B (t → Hc) < 3.4 (2.3) x 10^-4.