Search for a Dark Higgs produced in association with inelastic Dark Matter at the Belle II experiment

Abstract Inelastic dark matter models that feature two dark matter particles and a massive dark photon can reproduce the observed relic dark matter density without violating cosmological limits. The mass splitting between two dark matter particles chi1 and chi2 with mchi2 > mchi1 is induced by a dark Higgs field and a corresponding dark Higgs boson h'. We present a search for dark matter in events with two vertices, at least one of which must be displaced, and missing energy. Based on a sample corresponding to an integrated luminosity of 364fb-1 collected at Belle II, we observe no evidence for signal. We set upper limits on the product of branching fractions, where $x^+x^-$ indicates $\mu^+\mu^-, \pi^+\pi^-$, or $K^+K^-$, as functions of h' mass and lifetime at the level of 1e-1 fb. We set model-dependent upper limits on the dark Higgs mixing angle at the level of 1e-5 and on the dark photon kinetic mixing parameter at the level of 1e-3. This is the first search for dark Higgs bosons in association with inelastic dark matter.
Tags Run 1
Working group Dark Sector and Low Multiplicity
Principal authors @  Giacomo De Pietro, Patrick Ecker, Jonas Eppelt, Torben Ferber, Pablo Goldenzweig
Reference Phys.Rev.Lett. 135, 131801 (2025), DOI: 10.1103/37w5-glpp
Document BELLE2-PUB-DRAFT-2024-022
Links arXiv:2505.09705, Inspire, PRL
Bibtex 
@article{Belle-II:2025bhd,
    author = "Adachi, I. and others",
    collaboration = "Belle-II",
    title = "{Search for a Dark Higgs Boson Produced in Association with Inelastic Dark Matter at the Belle II Experiment}",
    eprint = "2505.09705",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2025-015, KEK Preprint 2025-14",
    doi = "10.1103/37w5-glpp",
    journal = "Phys. Rev. Lett.",
    volume = "135",
    number = "13",
    pages = "131801",
    year = "2025"
}