Search for the production of dark Higgs in the framework of Mono-Z$^{\prime}$ portal at the FCC-ee simulated electron-positron collisions at $\sqrt{s} = 240$ GeV

TL;DR

This work investigates the production of a light dark Higgs $h_D$ in the Mono-Z' portal at the FCC-ee with $\sqrt{s}=240$ GeV. Using simulated $e^+e^-$ collisions, the authors develop a recoil-mass–based, cut-based analysis of the $\mu^+\mu^-$ plus missing-energy final state and perform a shape-based likelihood to set 95% CL limits on $\sigma\times\mathrm{BR}(Z'\to\mu\mu)$ as a function of $M_{h_D}$, while evaluating discovery prospects. They find that a $5\sigma$ discovery is feasible for $M_{h_D}$ above $20$ GeV with $L=10.8~\mathrm{ab}^{-1}$, and that the $20$–$80$ GeV range can be excluded at 95% CL in the absence of a signal. Overall, the FCC-ee demonstrates significant potential to probe a light $Z'$–dark-Higgs sector beyond current LHC and LEP constraints, particularly in the muon channel for the chosen couplings $g_l=0.003$, $g_D=1.0$.

Abstract

In the present work, we study the possible production of the dark Higgs boson ($h_{D}$) candidates, which originated from a simplified-model scenario based on the Mono-Z$^{\prime}$ model, in association with a neutral gauge boson (Z$^{\prime}$). This study has been performed by studying events with dimuon plus missing transverse energy produced in the simulated electron-positron collisions at the foreseen Future Circular Collider in the Electron-Positron collision mode (FCC-ee), operating at 240 GeV center of mass energy and integrated luminosity of 10.8 ab$^{-1}$. In case no new physics has been discovered, we set upper limits at a 95\% confidence level on the mass of the dark Higgs.

Figures (8)

• Figure 1: Feynman diagram of the DH model, adapted from Ref. R1
• Figure 2: The measured dimuon invariant mass spectrum, after applying pre-selection summarized in table \ref{['cuts']}, for the estimated SM backgrounds and different choices of neutral gauge boson (Z$^{\prime}$) masses generated based on the DH scenario, with $\texttt{g}_{l} = 0.003$ and $\texttt{g}_{D} = 1.0$.
• Figure 3: The recoil mass spectrum, for events passing the pre-selection summarized in table \ref{['cuts']}, for the estimated SM backgrounds and different choices of the dark Higss masses generated based on the DH simplified model, with the coupling constants $\texttt{g}_{l} = 0.003$ and $\texttt{g}_{D} = 1.0$.
• Figure 4: The distributions of three variables for dimuon events, where each muon passes the low $p_T$ muon ID discussed in the pre-selection in table \ref{['section:AnSelection']}. The four variables are $|E^{miss} - E^{\mu^{+}\mu^{-}}|/E^{\mu^{+}\mu^{-}}$\ref{['figure:ptdiff']}, $\Delta\phi_{\mu^{+}\mu^{-},\vec{E}^{miss}}$\ref{['figure:deltaphi']}, $\text{cos}(\text{Angle}_{3D})$\ref{['figure:3Dangle']}, and $\Delta R(\mu^{+}\mu^{-})$\ref{['figure:deltar']}. The model corresponds to the DH scenario with two different values of $Z^{\prime}$ ($M_{Z^{\prime}}=$ 30 and 70 GeV) and SM backgrounds. The vertical dashed lines correspond to the chosen cut value for each variable.
• Figure 5: Distributions of the N-1 efficiencies plotted against the transverse momentum of the leading reconstructed muon ($p^{\mu}_{T}$) for the following cuts; $|E^{miss} - E^{\mu^{+}\mu^{-}}|/E^{\mu^{+}\mu^{-}} < 0.4$\ref{['eff2']}, $\Delta\phi_{\mu^{+}\mu^{-},\vec{E}^{\text{miss}}} > 3.0$\ref{['eff3']}, $\Delta R(\mu^{+}\mu^{-}) < 1.7$\ref{['eff4']}, and $\text{cos}(\text{Angle}_{3D}) < -0.8$\ref{['eff5']} for the signal in the DH scenario with $M_{Z^{\prime}}= 30$ GeV and for the SM backgrounds.