UC Irvine

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 $$\mathcal {B}(t\rightarrow Hc)<3.3\,(3.8) \times 10^{-4}$$ 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 $$H\rightarrow b\bar{b}$$ H → b b ¯ and $$H\rightarrow \gamma \gamma $$ H → γ γ decays, as well as $$H\rightarrow \tau ^{+}\tau ^{-}$$ H → τ + τ - decays with one or two hadronically decaying $$\tau $$ τ -leptons, yields observed (expected) upper limits on the branching ratios of $$\mathcal {B}(t\rightarrow Hu)<2.6\,(1.8) \times 10^{-4}$$ B ( t → H u ) < 2.6 ( 1.8 ) × 10 - 4 and $$\mathcal {B}(t\rightarrow Hc)<3.4\,(2.3) \times 10^{-4}$$ B ( t → H c ) < 3.4 ( 2.3 ) × 10 - 4 .