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Abstract

A search for a new heavy scalar particle X decaying into a Standard Model (SM) Higgs boson and a new singlet scalar particle S is presented. The search uses a protonproton (pp) collision data sample with an integrated luminosity of 140 fb−1 recorded at a centre-of-mass energy of √ s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The most sensitive mass parameter space is explored in X mass ranging from 500 to 1500 GeV, with the corresponding S mass in the range 200–500 GeV. The search selects events with two hadronically decaying τ -lepton candidates from H → τ +τ − decays and one or two light leptons (ℓ = e, µ) from S → V V (V = W, Z) decays while the remaining V boson decays hadronically or to neutrinos. A multivariate discriminant based on event kinematics is used to separate the signal from the background. No excess is observed beyond the expected SM background and 95% confdence level upper limits between 72 fb and 542 fb are derived on the cross-section σ(pp → X → SH) assuming the same SM-Higgs boson-like decay branching ratios for the S → V V decay. Upper limits on the visible cross-sections σ(pp → X → SH → WW τ τ ) and σ(pp → X → SH → ZZτ τ ) are also set in the ranges 3–26 fb and 6–33 fb, respectively.