Search for a new pseudoscalar decaying into a pair of muons in events with a top-quark pair at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ 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 $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ $pp$ collision data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of $139\text{}\text{}{\mathrm{fb}}^{-1}$. 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\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: $pp\to t\overline{t}a$ and $pp\to t\overline{t}$ with $t\to {H}^{±}b$, ${H}^{±}\to {W}^{±}a$, where $a\to \mu \mu $, in the mass ranges $15\text{}\text{}\mathrm{GeV}<{m}_{a}<72\text{}\text{}\mathrm{GeV}$ and $120\text{}\text{}\mathrm{GeV}\le {m}_{{H}^{±}}\le 160\text{}\text{}\mathrm{GeV}$.