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Free, publicly-accessible full text available April 17, 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 energycollider. The data sample was collected at and just below the center-of-mass energies of theandresonances and has an integrated luminosity of, corresponding toevents. We search for production of the HNL (denoted) in the decayfollowed by its decay via. The search focuses on the parameter-space region in which the HNL is long-lived, so that theoriginate 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 theneutrino as a function of the HNL mass.
Published by the American Physical Society 2024 Free, publicly-accessible full text available June 1, 2025 -
Free, publicly-accessible full text available March 1, 2025
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We measure the branching fraction of the decayusing data collected with the Belle II detector. The data contain 387 millionpairs produced incollisions at theresonance. We reconstructdecays from an analysis of the distributions of theenergy and thehelicity angle. We determine the branching fraction to be, in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two.
Published by the American Physical Society 2024 Free, publicly-accessible full text available June 1, 2025 -
Free, publicly-accessible full text available December 1, 2024
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Free, publicly-accessible full text available December 1, 2024
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We report on a search for a resonancedecaying to a pair of muons inevents in themass range, usingof data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models ofbeyond the standard model: avector boson in themodel and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for themodel and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for themodel and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.
Published by the American Physical Society 2024 Free, publicly-accessible full text available June 1, 2025 -
A bstract We measure the cross section of
e +e − →η c J/ψ at the Υ(nS )(n = 1–5) on-resonance and 10.52 GeV off-resonance energy points using the full data sample collected by the Belle detector with an integrated luminosity of 955 fb− 1. We also search for double charmonium production ine +e − →η c J/ψ via initial state radiation near theη c J/ψ threshold. No evident signal of the double charmonium state is found, but evidence for thee +e − → η c J/ψ process is found with a statistical significance greater than 3. 3σ near theη c J/ψ threshold. The average cross section near the threshold is measured and upper limits of cross sections are set for other regions.