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Abstract

A search is presented for flavour-changing neutralcurrent 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 tt ¯production, one top quark decays via t → H q. 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 → H q branching ratios at 95 % confidence level, amounting to observed (expected) limits of B(t → H u) < 2.8 (3.0) × 10−4 and B(t → Hc) < 3.3 (3.8) × 10−4. Combining this search with other searches fortHq flavour-changing neutral-current interactions previously conducted by ATLAS, considering H → bb¯ and H → γ γ 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 → H u) < 2.6 (1.8)×10−4 and B(t → Hc) < 3.4 (2.3) × 10−4.