A search for the resonant production of a heavy scalar $X$ decaying into a Higgs boson and a new lighter scalar $S$, through the process $X \to S(\to bb) H(\to \gamma\gamma)$, where the two photons are consistent with the Higgs boson decay, is performed. The search is conducted using an integrated luminosity of 140 fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The search is performed over the mass range 170 $\leq$$m_{X}$$\leq$ 1000 GeV and 15 $\leq$$m_{S}$$\leq$ 500 GeV. Parameterised neural networks are used to enhance the signal purity and to achieve continuous sensitivity in a domain of the ($m_{X}$, $m_{S}$) plane. No significant excess above the expected background is found and 95% CL upper limits are set on the cross section times branching ratio, ranging from 39 fb to 0.09 fb. The largest deviation from the background-only expectation occurs for ($m_{X}$, $m_{S}$) = (575, 200) GeV with a local (global) significance of 3.5 (2.0) standard deviations.
Number of events for the 2 b-jet process category obtained from a background-only fit to data in the signal region and sideband. In the signal region the yield in the most signal-like bin of the PNN distribution, parameterised to the signal point mass (mX, mS) = (250, 100) GeV, is also provided. The uncertainties are symmetrised around the central value. The uncertainty in the total background is calculated taking correlations between the individual contributions into account. For the single Higgs boson processes, 'Other' includes the following production modes: VBF, WH, tHq, and tHW.
Number of events for the 1 b-jet process category obtained from a background-only fit to data in the signal region and sideband. In the signal region the yield in the most signal-like bin of the PNN distribution, parameterised to the signal point mass (mX, mS) = (1000, 70) GeV, is also provided. The uncertainties are symmetrised around the central value. The uncertainty in the total background is calculated taking correlations between the individual contributions into account. For the single Higgs boson processes, 'Other' includes the following production modes: VBF, WH, tHq, and tHW.
Expected and observed 95% CL upper limits on the signal cross section times the branching ratio of X → SH → b b $\gamma \gamma$ shown in the (mX, mS) plane, for each evaluated point. The values of the expected limit one and two standard deviations from the nominal value are also shown. The band at mS = 125 GeV is not included as these signal points are equivalent to those already probed in Phys. Rev. D 106 (2022) 052001.
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