Search for $B \to \mu \nu_{\mu}$ with Inclusive Tagging at Belle and Belle II

Sumitted to PubDB: 2025-06-18

Category: Physics / performance paper

Tags: Run 1 Belle

Principal Authors Florian Bernlochner, Jochen Dingfelder, Daniel Jacobi, Peter Lewis, Markus Prim
Date 2026-06-23
Working group Semileptonic and Missing Energy Decay
Belle II Number BELLE2-PUB-PH-2025-020
Abstract We report a measurement of the branching fraction for the leptonic $B^+ \to \mu^+ \nu_\mu$ decay. This work presents the first $B^+ \to \mu^+ \nu_\mu$ result using Belle~II data, an updated Belle measurement superseding the previous result, and their combination, yielding the most precise search to date. The analysis is based on \SI{1.1}{\per\atto\barn} of \(e^+e^-\) collision data collected at a center-of-mass energy of \SI{10.58}{\giga\eV} with the Belle and Belle~II detectors at the KEKB and SuperKEKB colliders, respectively. An inclusive tagging approach is employed to enhance sensitivity, exploiting the back-to-back kinematics of \(\Upsilon(4S) \to B\bar{B}\) decays to derive the signal-side \(B\) direction and boost the signal muon into its rest frame. We measure \(\mathcal{B}(B^+ \to \mu^+ \nu_\mu) = (4.36 \pm 1.89 \pm 1.01) \times 10^{-7}\), where the first uncertainty is statistical and the second systematic. The observed significance relative to the background-only hypothesis is 2.35 standard deviations. Given the limited significance, we set 90\% confidence-level upper limits of \(\mathcal{B}(B^+ \to \mu^+ \nu_\mu) < 6.25 \times 10^{-7}\) using a frequentist approach and \(< 7.13 \times 10^{-7}\) using a Bayesian approach. These are the most stringent limits to date. The result is further interpreted as an exclusion in the parameter space of type~II and type~III two-Higgs-doublet models, extending the reach of previous Belle constraints. We search for stable sterile neutrinos with masses \(m_N \in [0,1.5]\,\mathrm{GeV}\). No signal is observed, and the resulting exclusion on the squared mixing parameter \(|U_{\mu N}|^2\) provides improvement over previous Belle limits. We also measure the partial branching fraction of semileptonic \(B \to X_u \ell \nu_\ell\) decays with \(p_\mu^B > \SI{2.2}{\giga \eV}\), obtaining \(\Delta\mathcal{B}(B \to X_u \ell \nu_\ell) = (0.286 \pm 0.005 \pm 0.031) \times 10^{-3} \). Finally, we present a model-dependent study of weak annihilation decays using the muon momentum spectrum. Modeling the weak annihilation contribution as a combination of flat and peaking components with a single floating fraction, we observe a signal of 3.33 standard deviations above the background-only hypothesis in regions where the distribution resembles that of \(B \to X_u \ell \nu_\ell\) decays. Elsewhere in parameter space, the fitted weak annihilation contribution is found to be much smaller.
ArXiv https://arxiv.org/abs/2602.09800
Inspire https://inspirehep.net/literature/3118119
Bibtex 
@article{Belle-II:2026flt,
    author = "Abumusabh, Merna and others",
    collaboration = "Belle-II, Belle",
    title = "{Study of $B^+ \to μ^+ ν_μ$ decays at Belle and Belle II}",
    eprint = "2602.09800",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint: 2026-001 KEK Preprint: 2025-38",
    month = "2",
    year = "2026"
}
Journal Phys.Rev.D
Preprint Number 2026-001

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