Search for a new heavy scalar particle decaying into a Higgs boson and a new scalar singlet in final states with one or two light leptons and a pair of τ-leptons with the ATLAS detector

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A bstract 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 proton-proton ( pp ) collision data sample with an integrated luminosity of 140 fb − 1 recorded at a centre-of-mass energy of √s 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 → VV ( 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% confidence 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 → VV decay. Upper limits on the visible cross-sections σ ( pp → X → SH → WWττ ) and σ ( pp → X → SH → ZZττ ) are also set in the ranges 3–26 fb and 6–33 fb, respectively.