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 ( $$q=u,c$$ 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 W bosons, two Z Z bosons or a τ ⁺τ ⁻ 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 t t ¯ and Ht production. For the tt̄ t t ¯ production, one top quark decays via t→ Hq t → H q . The proton–proton collision data set analysed amounts to (140\,fb⁻¹) ( 140 fb - 1 ) at (√s=13\,TeV) ( s = 13 TeV ) . No significant excess beyond Standard Model expectations is observed and upper limits are set on the t→ Hq t → H q branching ratios at 95 % confidence level, amounting to observed (expected) limits of 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 . Combining this search with other searches for $$tHq$$ tHq flavour-changing neutral-current interactions previously conducted by ATLAS, considering H→ bb̄ H → b b ¯ and H→ γ γ H → γ γ decays, as well as H→ τ ⁺τ ⁻ H → τ + τ - decays with one or two hadronically decaying τ τ -leptons, yields observed (expected) upper limits on the branching ratios of B(t→ Hu)<2.6\,(1.8) × 10⁻⁴ B ( t → H u ) < 2.6 ( 1.8 ) × 10 - 4 and B(t→ Hc)<3.4\,(2.3) × 10⁻⁴ B ( t → H c ) < 3.4 ( 2.3 ) × 10 - 4 .