Measurements of the mass difference m(B0) - m(B+) and the energy dependence of the cross-section ratio sigma(e+e- -> B0 Bbar0) / sigma(e+e- -> B+ B-) at Belle and Belle II

Journal of High Energy Physics. 2026. Vol. 05. Article 175.

Bodrov D., Pakhlova G., Aushev Т., Abumusabh М.

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(B0) - m(B+) = (0.495 +/- 0.024 +/- 0.005) MeV/c^2. The results are based on a simultaneous fit to the variable Mbar_bc, which is related to the B momentum, for B0 and B+ candidates; and to the energy dependence of R = sigma(e+ e- -> B0 Bbar0) / sigma(e+ e- -> B+ B-), which is measured using changes in the average center-of-mass energy over the data taking periods. The phase-space hypothesis R = (pB0/pB+)^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 R in a broader energy range than covered by the direct measurement and extract the energy dependence of R in the range from the B Bbar threshold up to 10.59 GeV. We interpret the results using a phenomenological model and constrain the parameters of the B Bbar potential in the isovector channel.

Keywords: B physics, Quarkonium, Spectroscopy

Publication based on the results of: Search for New physics in the experiments on e+e- colliders (2025)