Search for a resonance decaying into a scalar particle and a Higgs boson in final states with leptons and two photons in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for a hypothetical heavy scalar particle, $X$, decaying into a singlet scalar particle, $S$, and a Standard Model Higgs boson, $H$, using 140 fb${}^{-1}$ of proton-proton collision data at the centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the LHC is presented. The explored mass range is $300\le m_X\le 1000$ GeV and $170\le m_S\le 500$ GeV. The signature of this search is one or two leptons ($e$ or $\mu$) from the decay of vector bosons originating from the $S$ particle, $S\to W^{\pm }{W}^{\mp }\mathrm{/}ZZ$, and two photons from the Higgs boson decay, $H\to \gamma \gamma$. No significant excess is observed above the expected Standard Model background. The observed (expected) upper limits at the 95% confidence level on the cross-section for $gg\to X\to SH$, assuming the same $S\to WW\mathrm{/}ZZ$ branching ratios as for a SM-like heavy Higgs boson, are between 530 (800) fb and 120 (170) fb.