Exploring the Discovery Reach for a 95 GeV Scalar in Future $e^+e^-$ Collisions

TL;DR

The paper investigates the feasibility of discovering a 95 GeV scalar hinted by LEP and LHC data at future $e^+e^-$ colliders. It analyzes associated production with a Z boson, ZS, with Z decaying to a muon pair and S decaying to bottom quarks, using the recoil-mass method and a deep neural network to separate signal from Standard Model backgrounds, with the S–Higgs mixing captured by $\kappa_Z$. The results show that at $\sqrt{s}=250$ GeV or 200 GeV with an integrated luminosity of $5~\mathrm{ab}^{-1}$, a 5σ discovery is achievable for values of $\kappa_Z$ around 0.1, and the DNN significantly boosts significance and expands the reach in the $\kappa_Z$–Br$(S\rightarrow bb)$ plane, consistent with LEP hints. The study maps discovery potential for planned facilities (CEPC, ILC, CLIC, FCC-ee) and notes that including additional Z decay channels could further enhance sensitivity and shorten the path to discovery.

Abstract

The observed indications for a new scalar resonance with a mass around 95\,GeV, initially reported by LEP and supported by CMS and ATLAS in di-photon, $ττ$, and $W^+ W^-$ channels, motivate exploring its discovery potential at future electron-positron colliders. This study focuses on the production of the new scalar ($S$) via $e^+ e^- \rightarrow ZS $ with $Z \rightarrow μ^+ μ^- $ and $S \rightarrow b \bar{b}$ and optimizes the signal recognition using the recoil-mass method. By employing deep neural networks for signal-background discrimination, we demonstrate that a 95\,GeV scalar, mixing with the Standard Model Higgs by an angle of $\sim$0.1, can be observed with a 5$σ$ significance at $\sqrt{s}$ = 250\,GeV or 200\,GeV with 5~ab$^{-1}$ of integrated luminosity.

Figures (2)

• Figure 1: Recoil-mass distributions for the SM background (dashed) and the combined signal plus background (solid) for a scalar mass of $m_{S} = 95.5\,\mathrm{GeV}$ at $\sqrt{s} = 200\,\mathrm{GeV}$ (blue) and $\sqrt{s} = 250~\mathrm{GeV}$ (red) with an integrated luminosity of $\mathcal{L} = 500\,\mathrm{fb}^{-1}$.
• Figure 2: (a) Signal significance as a function of integrated luminosity for $m_{S} = 95.5\,\mathrm{GeV}$ at $\sqrt{s} = 250\,\mathrm{GeV}$ (red) and $200~\mathrm{GeV}$ (blue) within the recoil mass window $93.5\,\mathrm{GeV} \leq M_{\rm recoil} \leq 97.5~\mathrm{GeV}$. (b) Discovery region for a 95 GeV scalar in the $\kappa_Z$–Br($S \to b\bar{b}$) plane at $\sqrt{s} = 250\,\mathrm{GeV}$ and $\mathcal{L} = 5\,\mathrm{ab}^{-1}$, with the LEP-preferred region shown in red, the SM-like Higgs branching ratio as a blue line, and the gray area is excluded by the di-photon signal strength.