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 ^+ ^- 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 t t ¯ and Ht production. For the tt 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) B (t → H u) 2. 8 (3. 0) × 10 - 4 and B (t Hc) 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 bb H → b b ¯ and H H → γ γ decays, as well as H ^+ ^- 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) B (t → H u) 2. 6 (1. 8) × 10 - 4 and B (t Hc) B (t → H c) 3. 4 (2. 3) × 10 - 4.