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-quarkpair, 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 atthe LHC, corresponding to an integrated luminosity of $139~\mathrm{fb}^{-1}$.The search targets the final state where only one top quark decays to anelectron or muon, resulting in a signature with three leptons $e\mu\mu$ and$\mu\mu\mu$. No significant excess of events above the Standard Modelexpectation is observed and upper limits are set on two signal models: $pp\rightarrow t\bar{t}a$ and $pp \rightarrow t\bar{t}$ with $t \rightarrow H^\pmb$, $H^\pm \rightarrow W^\pm a$, where $a\rightarrow\mu\mu$, in the mass ranges$15$~GeV~$ < m_a < 72$~GeV and $120$~GeV~$ \leq m_{H^{\pm}} \leq 160$~GeV.