Abstract
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≤mX≤1000 GeV and 170≤mS≤500 GeV. The signature of this search is one or two leptons (e or μ) from the decay of vector bosons originating from the S particle, S→W±W∓/ZZ, and two photons from the Higgs boson decay, H→γγ. 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→X→SH, assuming the same S→WW/ZZ branching ratios as for a SM-like heavy Higgs boson, are between 530 (800) fb and 120 (170) fb.