Search for a new pseudoscalar decaying into a pair of muons in events with a top-quark pair at √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 √𝑠 =13  TeV 𝑝⁢𝑝 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 𝑒⁢𝜇⁢𝜇 and 𝜇⁢𝜇⁢𝜇. No significant excess of events above the Standard Model expectation is observed and upper limits are set on two signal models: 𝑝⁢𝑝→𝑡⁢¯𝑡⁢𝑎 and 𝑝⁢𝑝→𝑡⁢¯𝑡 with 𝑡→𝐻^±⁢𝑏, 𝐻^±→𝑊^±⁢𝑎, where 𝑎→𝜇⁢𝜇, in the mass ranges 15  GeV < 𝑚_𝑎 < 72  GeV and 120  GeV ≤𝑚_𝐻±  ≤ 160  GeV.