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Testing a 95 GeV Scalar at the CEPC with Machine Learning

Yabo Dong, Manqi Ruan, Kun Wang, Haijun Yang, Jingya Zhu

Abstract

Several possible excesses around 95 GeV hint at an additional light scalar beyond the Standard Model. We examine the capability of the CEPC to test this hypothesis in the Higgsstrahlung channel $e^{+}e^{-} \to ZS$ with $Z \toμ^{+}μ^{-}$ and $S\toτ^{+}τ^{-}$. Full detector simulation shows that the optimal center-of-mass energy to study the 95 GeV light scalar is 210 GeV. A deep neural network classifier reduces the luminosity required for discovery by half. At $L = 20~\mathrm{ab}^{-1}$, the CEPC's $5σ$ sensitivity to the signal strength $μ_{ττ}^{ZS}$ reaches 0.016 and 0.020 for $\sqrt{s} =$ 210 GeV and 240 GeV, respectively. The corresponding thresholds for a 5% precision measurement are $μ_{ττ}^{ZS} > 0.10$ and $>0.12$. At $\sqrt{s}=$ 210 GeV (240 GeV), $5σ$ coverage of all N2HDM-Flipped samples with $χ^2_{h_{95}}<7.82$ requires $L=800\ \mathrm{fb}^{-1}$ (1.22 $\mathrm{ab}^{-1}$). These results establish a 210 GeV run, augmented by machine-learning selection, as the most efficient strategy to confirm or refute the 95 GeV excess at future lepton colliders.

Testing a 95 GeV Scalar at the CEPC with Machine Learning

Abstract

Several possible excesses around 95 GeV hint at an additional light scalar beyond the Standard Model. We examine the capability of the CEPC to test this hypothesis in the Higgsstrahlung channel with and . Full detector simulation shows that the optimal center-of-mass energy to study the 95 GeV light scalar is 210 GeV. A deep neural network classifier reduces the luminosity required for discovery by half. At , the CEPC's sensitivity to the signal strength reaches 0.016 and 0.020 for 210 GeV and 240 GeV, respectively. The corresponding thresholds for a 5% precision measurement are and . At 210 GeV (240 GeV), coverage of all N2HDM-Flipped samples with requires (1.22 ). These results establish a 210 GeV run, augmented by machine-learning selection, as the most efficient strategy to confirm or refute the 95 GeV excess at future lepton colliders.

Paper Structure

This paper contains 6 sections, 9 equations, 5 figures, 2 tables.

Table of Contents

  1. 1. Introduction.
  2. 2. Monte Carlo simulation.
  3. 3. Machine learning techniques.
  4. 4. N2HDM-Flipped.
  5. 5. Conclusions and outlook.
  6. Acknowledgement.

Figures (5)

  • Figure 1: Recoil mass distributions for the signal process $ZS$ with $M_S = 95.5$ (normalized to 0.2), backgrounds $Z\tau\tau$ and $Zjj$ (normalized to 1), and the total (signal + background). The lower panel shows the ratio of the total to the background.
  • Figure 2: Signal significance $\mathcal{Z}$ in the $M_S$ versus $\sqrt{s}$ plane, assuming an integrated luminosity of 500 fb$^{-1}$. The red star indicates the benchmark point at $M_S = 95.5$ GeV and $\sqrt{s} = 210$ GeV.
  • Figure 3: Integrated luminosity required to reach $5\sigma$ significance at the benchmark point, as a function of the signal efficiency. Solid (dotted) curves correspond to $\sqrt{s} = 210\ (240)$ GeV. Colors denote different selection strategies: baseline (before applying ML classifiers, green), DNN (red), GBDT (yellow), and XGBoost (blue).
  • Figure 4: CEPC sensitivity in the $\mathrm{Br}(S\to\tau^+\tau^-)$ versus $C_{SZZ}$ planes. The left panel shows the expected $2\sigma$ and $5\sigma$ coverage, and the right panel shows the relative statistical precision on the signal yield. Results are presented for $\sqrt{s}=210$ GeV (solid) and $\sqrt{s}=240$ GeV (dashed), assuming an integrated luminosity of $L=20~\mathrm{ab}^{-1}$.
  • Figure 5: The coverage ability of CEPC for surviving samples in the $\mu_{\tau\tau}$ versus $\mu_{\gamma\gamma}$ plane at $\sqrt{s}=210$ GeV (left panel) and $\sqrt{s}=240$ GeV (right panel). CEPC with $L=100,~200,~800 {~\rm fb}^{-1},~\mathrm{and}~1.22{~\rm ab}^{-1}$ can cover the red, yellow, blue, and green samples at the $5\sigma$ level, respectively. All the surviving samples can be covered at $5\sigma$ for $\sqrt{s} = 210$ GeV with $L=800{~\rm fb}^{-1}$ and $\sqrt{s} = 240$ GeV with $L=1.22{~\rm ab}^{-1}$. The red star marks the best-fit point. The green and purple shaded bands indicate the experimental $1\sigma$ ranges for the $\tau^+\tau^-$ and $\gamma\gamma$ channels, respectively.