Precise measurement of the mass difference between $B^0$ and $B^+$ mesons

Abstract Using data samples collected by the Belle and Belle II experiments at the $\Upsilon(4S)$ resonance with integrated luminosities of $571 fb^{-1}$ and $365 fb^{-1}$, respectively, we measure the pseudoscalar $B$-meson mass difference to be $m(B^0)-m(B^+) = (0.495\pm0.024\pm0.005) MeV/c^2$. The results are based on a simultaneous fit to the variable $\tilde{M}_{bc}$, which is related to the $B$ momentum, for $B^0$ and $B^+$ candidates; and to the energy dependence of ${\cal R}=\sigma(e^+e^-\to B^0\bar{B}^0) / \sigma(e^+e^-\to B^+B^-)$, which is measured using changes in the average center-of-mass energy over the data taking periods. The phase-space hypothesis ${\cal R}=(p_{B^0}/p_{B^+})^3$, upon which previous measurements rely, is strongly disfavored by our fit; the measured mass-difference value for the phase-space hypothesis also differs significantly from our measurement. We constrain ${\cal R}$ in a broader energy range than covered by the direct measurement and extract the energy dependence of ${\cal R}$ in the range from the $B\bar{B}$ threshold up to $10.59 GeV$. We interpret the results using a phenomenological model and constrain the parameters of the $B\bar{B}$ potential in the isovector channel.
Tags Run 1 Belle
Working group Quarkonium
Principal authors @  Alexander Bondar, Roman Mizuk
Reference submitted to JHEP
Links arXiv:2511.15926, Inspire,
Bibtex 
@article{Belle-II:2025kfw,
    author = "Abumusabh, M. and others",
    collaboration = "Belle-II, Belle",
    title = "{Measurements of the mass difference $m(B^0)-m(B^+)$ and the energy dependence of the cross-section ratio $σ(e^+e^-\to B^0\bar{B}^0) / σ(e^+e^-\to B^+B^-)$ at Belle and Belle II}",
    eprint = "2511.15926",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2025-028, KEK Preprint 2025-35",
    month = "11",
    year = "2025"
}