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 $139{\mathrm{f}\mathrm{b}}^{-1}$. The search targets the final state where only one top quark decays to an electron or muon, resulting in a signature with three leptons $e\mu \mu$ and $\mu \mu \mu$. 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 $a\to \mu \mu$, in the mass ranges $15$ GeV $<m_a<72$ GeV and $120$ GeV $\le m_{H^{\pm }}\le 160$ GeV.