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 α-boson produced in events with a top-quark pair, where the α-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 $\to t\bar{t}$a and pp $\to t\bar{t}$ with $t \to H^{\pm}b, H^{\pm} \to W^{\pm}a$, where a $\to$ μμ, in the mass ranges 15 GeV $< m_{a} <$ 72 GeV and 120 GeV $\leq m_{H^{\pm}} \leq$ 160 GeV.