Measurements of the mass difference m(B0) − m(B ) and the energy dependence of the cross-section ratio at Belle and Belle II

Abstract

Using data samples collected by the Belle and Belle II experiments at the Υ(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/c2. The results are based on a simultaneous fit to the variable, which is related to the B momentum, for B0 and B+ candidates; and to the energy dependence of, which is measured using changes in the average center-of-mass energy over the data taking periods. The phase-space hypothesis, upon which previous measurements rely, is strongly disfavored by our fit; the measured mass-difference value for the phase-space hypothesis also differs..