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 searchuses a proton-proton ($pp$) collision data sample with an integrated luminosityof 140 fb$^{-1}$ recorded at a centre-of-mass energy of $\sqrt{s} = 13$ TeVwith the ATLAS detector at the Large Hadron Collider. The most sensitive massparameter space is explored in $X$ mass ranging from 500 to 1500 GeV, with thecorresponding $S$ mass in the range 200-500 GeV. The search selects events withtwo hadronically decaying $\tau$-lepton candidates from $H\to \tau^+\tau^-$decays and one or two light leptons ($\ell=e,\,\mu$) from $S\to VV$ ($V =W,\,Z$) decays while the remaining $V$ boson decays hadronically or toneutrinos. A multivariate discriminant based on event kinematics is used toseparate the signal from the background. No excess is observed beyond theexpected SM background and 95% confidence level upper limits between 72 fb and542 fb are derived on the cross-section $\sigma(pp\to X\to SH)$ assuming thesame SM-Higgs boson-like decay branching ratios for the $S\to VV$ decay. Upperlimits on the visible cross-sections $\sigma(pp\to X\to SH \to WW\tau\tau)$ and$\sigma(pp\to X\to SH \to ZZ\tau\tau)$ are also set in the ranges 3-26 fb and6-33 fb, respectively.