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.
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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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Free, publicly-accessible full text available February 1, 2025
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Free, publicly-accessible full text available February 1, 2025
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A bstract We report a search for the charged-lepton flavor violation in Υ(2
S ) →ℓ ∓τ ± (ℓ =e, μ ) decays using a 25 fb− 1Υ(2S ) sample collected by the Belle detector at the KEKBe +e − asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions ( ) at 90% confidence level. We obtain the most stringent upper limits:$$ \mathcal{B} $$ (Υ(2$$ \mathcal{B} $$ S )→ μ ∓τ ± )< 0. 23× 10− 6and (Υ(2$$ \mathcal{B} $$ S )→ e ∓τ ± )< 1. 12× 10− 6.Free, publicly-accessible full text available February 1, 2025 -
Free, publicly-accessible full text available December 1, 2024
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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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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