Search for a new pseudoscalar decaying into a pair of bottom and antibottom quarks in top-associated production in $\sqrt{s}$=13 TeV proton-proton collisions with the ATLAS detector

A search for a pseudoscalar $a$ produced in association with a top-quark pair, or in association with a single top quark plus a $W$ boson, with the pseudoscalar decaying into $b$-quarks ($a\to b\bar{b}$), is performed using the full Run 2 data sample using a dileptonic decay mode signature. The search covers pseudoscalar boson masses between 12-100 GeV and involves both the kinematic regime where the decay products of the pseudoscalar are reconstructed as two standard $b$-tagged small-radius jets, or merged into a large-radius jet due to its Lorentz boost. No significant excess relative to expectations is observed. Assuming a branching ratio BR($a\to b\bar{b}$)=100$\mathrm{\%}$, the range of pseudoscalar masses between 50 and 80 GeV is excluded at 95$\mathrm{\%}$ confidence level for a coupling of the pseudoscalar to the top quark of 0.5, while a coupling of 1.0 is excluded at 95$\mathrm{\%}$ confidence level for the masses considered, with the coupling defined as the strength modifier of the Standard Model Yukawa coupling.