Search for a dark photon and an invisible dark Higgs boson in $\mu^+ \mu^{\mathstrut -}$ and missing energy final states with the Belle II experiment

Abstract The dark photon $A^\prime$ and the dark Higgs boson $h^\prime$ are hypothetical particles predicted in many dark sector models. We search for the simultaneous production of $A^\prime$ and $h^\prime$ in the dark Higgsstrahlung process $e^{+}e^{-}\rightarrow A^\prime \, h^\prime$ with $A^\prime \rightarrow \mu^+\mu^-$ and $ h^\prime$ invisible in electron-positron collisions at a center-of-mass energy of 10.58 GeV collected by the Belle II experiment in 2019. With an integrated luminosity of 8.34 fb$^{-1}$, we observe no evidence for signal. We obtain exclusion limits at 90% Bayesian credibility in the range of 1.7--5.0 fb on the cross section and in the range of $1.7 \times10^{-8}$--$200 \times10^{-8}$ on the effective coupling $\varepsilon^2 \times \alpha_D$ for the $A^\prime$ mass in the range of 4.0 GeV/$c^2$ $< M_{A^\prime}< 9.7$ GeV/$c^2$ and for the $h^\prime$ mass $M_{h^\prime} < M_{A^\prime}$, where $\varepsilon$ is the mixing strength between the standard model and the dark photon and $\alpha_D$ is the coupling of the dark photon to the dark Higgs boson. Our limits are the first in this mass range.
Tags
Working group Dark Sector and Low Multiplicity
Principal authors @  Michel Bertemes, Marcello Campajola, Giacomo De Pietro, Enrico Graziani, Gianluca Inguglia
Reference Phys.Rev.Lett. 130, 071804 (2023), DOI: 10.1103/PhysRevLett.130.071804
Document BELLE2-PUB-PH-2022-011
Links arXiv:2207.00509, Inspire, PRL
Bibtex 
@article{Belle-II:2022jyy,
    author = "Abudin{\'e}n, F. and others",
    collaboration = "Belle-II",
    title = "{Search for a Dark Photon and an Invisible Dark Higgs Boson in {\ensuremath{\mu}}+{\ensuremath{\mu}}- and Missing Energy Final States with the Belle II Experiment}",
    eprint = "2207.00509",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2022-001, KEK Preprint 2022-6",
    doi = "10.1103/PhysRevLett.130.071804",
    journal = "Phys. Rev. Lett.",
    volume = "130",
    number = "7",
    pages = "071804",
    year = "2023"
}