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 √s = 13 TeV with the ATLAS detector

A bstract 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 ≤ m X ≤ 1000 GeV and 170 ≤ m S ≤ 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.