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

Abstract 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 $$\tau ^{+}\tau ^{-}$$ 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 $$t\bar{t}$$ t t ¯ and Ht production. For the $$t\bar{t}$$ t t ¯ production, one top quark decays via $$t\rightarrow Hq$$ t → H q . The proton–proton collision data set analysed amounts to $$({140}\,{\hbox {fb}^{-1}})$$ ( 140 fb - 1 ) at $$(\sqrt{s}={13}\,\hbox {TeV})$$ ( s = 13 TeV ) . No significant excess beyond Standard Model expectations is observed and upper limits are set on the $$t\rightarrow Hq$$ t → H q branching ratios at 95 % confidence level, amounting to observed (expected) limits of $$\mathcal {B}(t\rightarrow Hu)<2.8\,(3.0) \times 10^{-4}$$ B ( t → H u ) < 2.8 ( 3.0 ) × 10 - 4 and $$\mathcal {B}(t\rightarrow Hc)<3.3\,(3.8) \times 10^{-4}$$

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