Search for a heavy Higgs boson decaying into a Z boson and another heavy Higgs boson in the $$\ell \ell bb$$ and $$\ell \ell WW$$ final states in pp collisions at $$\sqrt{s}=13$$ $$\text {TeV}$$ with the ATLAS detector

A search for a heavy neutral Higgs boson, A, decaying into a Z boson and another heavy Higgs boson, H, is performed using a data sample corresponding to an integrated luminosity of 139 fb$$^{-1}$$ - 1 from proton–proton collisions at $$\sqrt{s} = 13$$ s = 13 $$\text {TeV}$$ TeV recorded by the ATLAS detector at the LHC. The search considers the Z boson decaying into electrons or muons and the H boson into a pair of b-quarks or W bosons. The mass range considered is 230–800 $$\text {GeV}$$ GeV for the A boson and 130–700 $$\text {GeV}$$ GeV for the H boson. The data are in good agreement with the background predicted by the Standard Model, and therefore 95% confidence-level upper limits for $$\sigma \times B(A\rightarrow ZH)\times B(H\rightarrow bb \; \text {or} \; H \rightarrow WW)$$ σ × B ( A → Z H ) × B ( H → b b or H → W W ) are set. The upper limits are in the range 0.0062–0.380 pb for the $$H\rightarrow bb$$ H → b b channel and in the range 0.023–8.9 pb for the $$H\rightarrow WW$$ H → W W channel. An interpretation of the results in the context of two-Higgs-doublet models is also given.