Search for a CP-odd Higgs boson decaying into a heavy CP-even Higgs boson and a Z boson in the ${\ell }^{+}{\ell }^{-}t\overline{t}$ and $\nu \overline{\nu }b\overline{b}$ final states using 140 fb −1 of data collected with the ATLAS detector

A search for a heavy CP-odd Higgs boson, A, decaying into a Z boson and a heavy CP-even Higgs boson, H, is presented. It uses the full LHC Run 2 dataset of pp collisions at $\sqrt{s}$ = 13 TeV collected with the ATLAS detector, corresponding to an integrated luminosity of 140 fb −1. The search for A → ZH is performed in the ${\ell }^{+}{\ell }^{-}t\overline{t}$ and $\nu \overline{\nu }b\overline{b}$ final states and surpasses the reach of previous searches in different final states in the region with m H > 350 GeV and m A > 800 GeV. No significant deviation from the Standard Model expectation is found. Upper limits are placed on the production cross-section times the decay branching ratios. Limits with less model dependence are also presented as functions of the reconstructed m( $t\overline{t}$ ) and m( $b\overline{b}$ ) distributions in the ${\ell }^{+}{\ell }^{-}t\overline{t}$ and $\nu \overline{\nu }b\overline{b}$ channels, respectively. In addition, the results are interpreted in the context of two-Higgs-doublet models.