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

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√=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.