Search for a resonance decaying into a scalar particle and a Higgs boson in the final state with two bottom quarks and two photons with 199 fb$^{-1}$ of data collected at $\sqrt{s}$=13 TeV and $\sqrt{s}$=13.6 TeV with the ATLAS detector

TL;DR

This ATLAS study searches for a resonant scalar X decaying to a lighter scalar S and a Higgs boson H, with H decaying to two photons and S to bb. Using Run 2 (13 TeV, 140 fb^-1) and Run 3 (13.6 TeV, 58.6 fb^-1) data, the analysis scans a wide mass plane (170 ≤ m_X ≤ 1000 GeV, 15 ≤ m_S ≤ 500 GeV) under the narrow-width assumption, employing parameterised neural networks trained across mass hypotheses and a data-driven background normalization in control regions. The combination yields no significant excess; observed 95% CL upper limits on σ(X→S(H→γγ)) range from 9 fb down to 0.06 fb, with substantial gains (~15–73%) over the prior Run-2 result thanks to Run-3 data, tighter selection, and improved ML training. These results tighten constraints on extended Higgs-sector models predicting X→SH cascades and demonstrate the efficacy of mass-parameterised ML discriminants and signal interpolation for broad parameter coverage.

Abstract

A search for the resonant production of a heavy scalar $X$ decaying into a Higgs boson and a lighter scalar $S$, through the process $X \rightarrow S (\rightarrow b \bar{b})H ( \rightarrow γγ)$, where the two photons are consistent with the Higgs boson decay, is performed. The search is conducted using integrated luminosities of 140 fb$^{-1}$ and 58.6 fb$^{-1}$ of proton-proton collision data at centre-of-mass energies of 13 TeV and 13.6 TeV respectively, recorded with the ATLAS detector at the Large Hadron Collider. The search is performed over the mass ranges of 170 $\leq$ $m_{X}$ $\leq$ 1000 GeV and 15 $\leq$ $m_{S}$ $\leq$ 500 GeV. No significant excess over the Standard Model background prediction is observed and limits at 95% confidence level are set on the cross-section times branching ratio $σ(X \rightarrow S (\rightarrow b \bar{b})H ( \rightarrow γγ))$ at 13 TeV, ranging from 9 fb to 0.06 fb.

Figures (3)

• Figure 1: Illustrative Feynman diagram for $X\to S({\to}b\bar{b})\xspace H({\to}\gamma\gamma)\xspace$ production via gluon--gluon fusion.
• Figure 2: Distributions of the PNN discriminant output after the profile-likelihood fit in the SRs. Figures (a) and (b) show the 1 $b$-tagged SR for $m_{X}\xspace\ = 1000~\text{Ge V}\xspace$ and $m_{S}\xspace = 70~\text{Ge V}\xspace$ using (a) Run-2 and (b) Run-3 data. Figures (c) and (d) show the 2 $b$-tagged SR for $m_{X}\xspace\ = 575~\text{Ge V}\xspace$ and $m_{S}\xspace = 200~\text{Ge V}\xspace$ using (c) Run-2 and (d) Run-3 data. The $\gamma\gamma$+jets category represents the sum of $\gamma\gamma$+jets, $\gamma$+jets and dijet processes. The shaded band represents the total systematic uncertainty after the profile-likelihood fit. The blue line represents the signal normalised using an arbitrary cross-section of 1 fb.
• Figure 3: (a) Expected and (b) observed 95% CL upper limits on the $X\to S({\to}b\bar{b})\xspace H({\to}\gamma\gamma)\xspace$ cross-section at 13 $\text{Te V}$, in the full ($m_{X}$, $m_{S}$) plane, including both the 1 $b$-tagged and 2 $b$-tagged categories.