Discovery Potential of Future Electron-Positron Colliders for a 95 GeV Scalar

TL;DR

This work investigates the discovery prospects for a 95 GeV scalar $S$ at future $e^+e^-$ colliders using the recoil-mass method in $e^+e^-\to ZS$ with $Z\to \mu^+\mu^-$ and $S\to b\bar{b}$. It demonstrates that a Deep Neural Network can substantially boost signal discrimination against the dominant SM background, reducing the required luminosity by a factor of 2–3 and enabling a $>5\sigma$ discovery for $\kappa_Z \ge 0.1$ at $\sqrt{s}=250$ GeV with ${\cal L}=5\,\text{ab}^{-1}$; including additional $S$ decay channels further improves sensitivity. The study provides detailed signal and background modeling (MadGraph5, Pythia8, Delphes CEPC) and a rigorous ML-based classification pipeline, reporting significant gains in discovery reach and emphasizing the role of $Z$-recoil tagging for model-independent tests of the $SZZ$ coupling. The findings support the capability of future $e^+e^-$ Higgs factories to test the LEP/CMS/ATLAS hints near $95$ GeV, with implications for singlet-Higgs scenarios and extended Higgs sectors such as 2HDM+S.

Abstract

The Large Electron Positron collider observed an indication for a new Higgs boson with a mass around $95$\,GeV-$100$\,GeV in the process $e^+e^-\to Z^*\to ZS$ with $S\to b\bar b$. The interest in this excess re-emerged with the di-photon signature at $\approx$\,95\,GeV at the Large Hadron Collider. In fact, a combined global significance of $3.4σ$ is obtained once $WW$ and $ττ$ signals are included in addition. In this article, we perform a feasibility study for discovering such a new scalar $S$ at future electron-positron colliders using the recoil-mass method applied to $e^{+} e^{-} \to ZS$ with $Z \rightarrow μ^{+} μ^{-}$ and $S \to b \bar{b}$. For this, we employ a Deep Neural Network to enhance the separation between the Standard Model background and the signal, reducing the required integrated luminosity necessary for discovery by a factor of two to three. As a result, an $SU(2)_L$ singlet Higgs with a mass of $\approx$\,95\,GeV can be observed with more than 5$σ$ significance at a 250\,GeV centre-of-mass energy collider with $5~ {\rm ab}^{-1}$ integrated luminosity if it has a mixing angle of at least $0.1$ with the Standard Model Higgs, which means that a discovery can be achieved within the whole 95\% confidence-level region preferred by Large Electron Positron excess. Furthermore, including more decay channels such as $S\to ττ$ and $Z\to e^+e^-$ further enhances the discovery potential of future $e^+e^-$ accelerators, like CEPC, CLIC, FCC-ee and ILC.

Figures (2)

• Figure 1: Recoil-mass distribution of the simulated SM background (dashed) and the simulated signal plus background (solid) for $m_{S} = 95.5$ GeV for $\sqrt{s}=200$ GeV (blue) and $\sqrt{s}=250$ GeV (red) at $\mathcal{L}= 500$fb$^{-1}$.
• Figure 2: (a) The significance as a function of luminosity for $m_{S} = 95.5$ GeV and $\sqrt{s}$ = 250 GeV (200 GeV) is shown in red (blue). We consider the number of events in the recoil mass window of 93.5 GeV -- 97.5 GeV before (after) the DNN has been applied as a dashed (solid) line. A cut of 0.965 (0.959) is applied on DNN response for $\sqrt{s}$ = 250 GeV (200 GeV). (b) Discovery region for a 95 GeV scalar in the $\kappa_Z$-Br($S\to b\bar{b}$) plane for an integrated luminosity of $\mathcal{L} = 5$ab$^{-1}$ at $\sqrt{s}=250$ GeV. The red region is currently preferred by the LEP measurement of Higgs-strahlung. The blue line indicates the Br($S\to b\bar{b}$) for a SM-like Higgs, and the gray region is excluded by the di-photon signal strength of the SM Higgs assuming that $S$ is an $SU(2)_L$ singlet.