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 $\sqrt{s}=13\mathrm{TeV}$ $pp$ collision data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of $139{\mathrm{fb}}^{-1}$. 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 $t\to H^{\pm }b$, $H^{\pm }\to W^{\pm }a$, where $a\to \mu \mu$, in the mass ranges $15\mathrm{GeV}<m_a<72\mathrm{GeV}$ and $120\mathrm{GeV}\le m_{H^{\pm }}\le 160\mathrm{GeV}$.

• Received 28 April 2023
• Accepted 18 September 2023

DOI:https://doi.org/10.1103/PhysRevD.108.092007

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