Search for a new pseudoscalar decaying into a pair of muons in events with a top-quark pair at sqrt(s) = 13 TeV with the ATLAS detector

A search for a new pseudoscalar a-boson produced in events with a top-quark pair, where the a-boson decays into a pair of muons, is performed using sqrt(s) = 13 TeV pp collision data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139 fb−1. The search targets the final state where only one top quark decays to an electron or muon, resulting in a signature with three leptons eμμ and μμμ. No significant excess of events above the Standard Model expectation is observed and upper limits are set on two signal models: pp → t¯ta and pp → t¯t with t → Hb, H → Wa, where a → μμ, in the mass ranges 15 GeV < ma < 72 GeV and 120 GeV ≤ mH ≤ 160 GeV.