Dark Higgs in Hidden Sector as a Probe for Dark Matter

TL;DR

This work evaluates the sensitivity of the FCC-ee to exotic Higgs decays h→ss where the long-lived scalar s decays to bb, via the Zh production mode at √s=240 GeV. Using the FCCAnalyses framework with MadGraph, Pythia, and Delphes, the study simulates h→ss→bbbb events, reconstructs displaced vertices, and applies a Z plus displaced-vertex selection to suppress backgrounds. Two scalar masses (20 and 60 GeV) and three mixing angles (sinθ=10⁻⁵,10⁻⁶,10⁻⁷) are explored, spanning decay lengths from ~1 mm to ~10 m, with optimal sensitivity near 0.3 m; BR$(h→ss)$ is set to ~10⁻⁴ via κ=0.001. Validation of generated quantities shows consistent scalar multiplicities, masses, and lifetimes, with mean lifetimes from simulations agreeing to within ~5% of theoretical expectations. The results indicate FCC-ee can probe LLPs over a broad lifetime and mass range, providing a direct test of hidden-sector Higgs portals and contributing to the broader search for dark-sector physics at future lepton colliders.

Abstract

This study presents a sensitivity analysis of exotic Higgs boson decays at the electron--positron stage of the Future Circular Collider (FCC-ee), performed within the FCCAnalyses framework. The analysis investigates Higgs boson production in association with a Z boson in electron--positron collisions at a center-of-mass energy of 240~GeV. The Higgs boson is assumed to decay into a pair of long-lived scalar particles, while the Z boson decays leptonically. A hadronic final state is considered, in which the long-lived scalars subsequently decay into bottom--antibottom quark pairs. The simulation chain is implemented using the FCCAnalyses framework, with event generation performed using \textsc{MadGraph} and \textsc{Pythia}, and detector effects modeled with \textsc{Delphes}. Displaced vertices arising from the decays of long-lived particles are reconstructed using the FCCAnalyses implementation of the \textsc{LCFIPlus} secondary vertex finding algorithm, enhanced with extended features such as customized track selection. The final event selection requires the reconstruction of a Z boson together with at least two displaced vertices. This strategy efficiently suppresses Standard Model backgrounds while retaining at least three expected signal events, including statistical uncertainties, over most of the explored parameter space. The study considers scalar masses of 20~GeV and 60~GeV and mixing angles of $10^{-5}$, $10^{-6}$, and $10^{-7}$. The results demonstrate that FCC-ee will be sensitive to long-lived scalars with decay lengths ranging from approximately 1~mm to 10~m, with optimal sensitivity around a decay length of 0.3~m.

Figures (8)

• Figure 1: Feynman diagram of the complete signal process $h \rightarrow ss \rightarrow b\bar{b}b\bar{b}$ produced at the FCC-ee via $e^{+}e^{-} \rightarrow Zh$, with the $Z$ boson decaying into $e^{+}e^{-}$ or $\mu^{+}\mu^{-}$.
• Figure 2: The number of generated scalars in each event for the signal sample $m_s = 20~\mathrm{GeV}$ and $\sin\theta = 1\times10^{-5}$.
• Figure 3: Generated mass of the scalars for signal samples with $\sin\theta = 1\times10^{-6}$ and masses $m_s = 20~\mathrm{GeV}$ and $m_s = 60~\mathrm{GeV}$.
• Figure 4: Generated decay lengths, $L_{xyz}^{\mathrm{gen}}$, for all signal samples.
• Figure 5: Distribution of the lifetimes for all signal samples.