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We report measurements of production cross sections for , , , , , , , , , , , , , , and in collisions at a center-of-mass energy near 10.58 GeV. The data were recorded by the Belle experiment, consisting of at 10.58 GeV and at 10.52 GeV. Production cross sections are extracted as a function of the fractional hadron momentum . The measurements are compared to Monte Carlo generator predictions with various fragmentation settings, including those that have increased fragmentation into vector mesons over pseudoscalar mesons. The cross sections measured for light hadrons are consistent with no additional increase of vector over pseudoscalar mesons. The charmed-meson cross sections are compared to earlier measurements—when available—including older Belle results, which they supersede. They are in agreement before application of an improved initial-state radiation correction procedure that causes slight changes in their shapes. Published by the American Physical Society2025more » « lessFree, publicly-accessible full text available March 1, 2026
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Free, publicly-accessible full text available January 1, 2026
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In the bottomonium sector, the hindered magnetic dipole transitions between P-wave states , , 1, 2, are expected to be severely suppressed according to the relativized quark model, due to the spin flip of the quark. Nevertheless, a recent model following the coupled-channel approach predicts the corresponding branching fractions to be enhanced by orders of magnitude. In this Letter, we report the first search for such transitions. We find no significant signals and set upper limits at 90% confidence level on the corresponding branching fractions: , and . These values help to constrain the parameters of the coupled-channel models. The results are obtained using a data sample taken around with the Belle detector at the KEKB asymmetric-energy collider. Published by the American Physical Society2025more » « lessFree, publicly-accessible full text available January 1, 2026
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We report the first evidence for the transition with a significance of 3.5 standard deviations. The decay branching fraction is measured to be , which is noticeably smaller than expected. We also set upper limits on transitions of , and , at the 90% confidence level. These results are obtained with a data sample collected near the resonance with the Belle detector at the KEKB asymmetric-energy collider. Published by the American Physical Society2024more » « lessFree, publicly-accessible full text available December 1, 2025
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We perform an angular analysis of the decay for the dielectron mass squared, , range of using the full Belle dataset in the and channels, incorporating new methods of electron identification to improve the statistical power of the dataset. This analysis is sensitive to contributions from right-handed currents from physics beyond the Standard Model by constraining the Wilson coefficients . We perform a fit to the differential decay rate and measure the imaginary component of the transversality amplitude to be , and the transverse asymmetry to be , with and fixed to the Standard Model values. The resulting constraints on the value of are consistent with the Standard Model within a confidence interval. Published by the American Physical Society2024more » « lessFree, publicly-accessible full text available October 1, 2025
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We report the results of the first search for Standard Model and baryon-number-violating two-body decays of the neutral mesons to and using of data collected at the resonance with the Belle detector at the KEKB asymmetric-energy collider. We observe no evidence of signal from any such decays and set 95% confidence-level upper limits on the products of and branching fractions for these two-body decays with in the range between and . Published by the American Physical Society2024more » « lessFree, publicly-accessible full text available August 1, 2025
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We measure the complete set of angular coefficients for exclusive decays ( , ). Our analysis uses the full Belle dataset with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the transition and the Cabibbo-Kobayashi-Maskawa matrix element . Using recent lattice QCD calculations for the hadronic form factors, we find using the Boyd-Grinstein-Lebed parametrization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter and investigate the differences of the electron and muon angular distributions. We find no deviation from standard model expectations. Published by the American Physical Society2024more » « lessFree, publicly-accessible full text available September 1, 2025
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We report the results of the first search for decays to the final state using of data collected at the resonance with the Belle detector at the KEKB asymmetric-energy collider. The results are interpreted in terms of both direct baryon-number-violating decay and oscillations which follow the standard model decay . We observe no evidence for baryon number violation and set the 95% confidence-level upper limits on the ratio of baryon-number-violating and standard model branching fractions to be and on the effective angular frequency of mixing in oscillations to be (equivalent to ). Published by the American Physical Society2024more » « lessFree, publicly-accessible full text available August 1, 2025
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We present a comprehensive study of decays using pairs collected with the Belle detector at the KEKB collider. This process is a suppressed charmless decay into two vector mesons and can exhibit interesting polarization and violation. The decay is observed for the first time with a significance of 7.9 standard deviations. We measure a branching fraction , a fraction of longitudinal polarization , and a time-integrated asymmetry , where the first uncertainties listed are statistical and the second are systematic. This is the first observation of and the first measurements of and for this decay. Published by the American Physical Society2024more » « lessFree, publicly-accessible full text available August 1, 2025
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We report a search for a heavy neutral lepton (HNL) that mixes predominantly with . The search utilizes data collected with the Belle detector at the KEKB asymmetric energy collider. The data sample was collected at and just below the center-of-mass energies of the and resonances and has an integrated luminosity of , corresponding to events. We search for production of the HNL (denoted ) in the decay followed by its decay via . The search focuses on the parameter-space region in which the HNL is long-lived, so that the originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the neutrino as a function of the HNL mass. Published by the American Physical Society2024more » « less