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

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 tt̄ and $Ht$ production. For the tt̄ 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→ Hu)<2.8\,(3.0) × 10⁻⁴ and B(t→ Hc)<3.3\,(3.8) × 10⁻⁴. Combining this search with other searches for $tHq$ flavour-changing neutral-current interactions previously conducted by ATLAS, considering H→ bb̄ 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) × 10⁻⁴ and B(t→ Hc)<3.4\,(2.3) × 10⁻⁴.