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  1. We report a search for a heavy neutral lepton (HNL) that mixes predominantly withντ. The search utilizes data collected with the Belle detector at the KEKB asymmetric energye+ecollider. The data sample was collected at and just below the center-of-mass energies of theϒ(4S)andϒ(5S)resonances and has an integrated luminosity of915fb1, corresponding to(836±12)×106e+eτ+τevents. We search for production of the HNL (denotedN) in the decayτπNfollowed by its decay viaNμ+μντ. The search focuses on the parameter-space region in which the HNL is long-lived, so that theμ+μoriginate 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 theτneutrino as a function of the HNL mass.

    Published by the American Physical Society2024 
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    Free, publicly-accessible full text available June 1, 2025
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  5. A<sc>bstract</sc>

    We report a search for the charged-lepton flavor violation in Υ(2S) →τ±(=e, μ) decays using a 25 fb1Υ(2S) sample collected by the Belle detector at the KEKBe+easymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions ($$ \mathcal{B} $$B) at 90% confidence level. We obtain the most stringent upper limits:$$ \mathcal{B} $$B(Υ(2S)→ μτ±)<0.23×106and$$ \mathcal{B} $$B(Υ(2S)→ eτ±)<1.12×106.

     
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    Free, publicly-accessible full text available February 1, 2025
  6. We measure the branching fraction of the decayBD0ρ(770)using data collected with the Belle II detector. The data contain 387 millionBB¯pairs produced ine+ecollisions at theϒ(4S)resonance. We reconstruct8360±180decays from an analysis of the distributions of theBenergy and theρ(770)helicity angle. We determine the branching fraction to be(0.939±0.021(stat)±0.050(syst))%, 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 Society2024 
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    Free, publicly-accessible full text available June 1, 2025
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  9. We report on a search for a resonanceXdecaying to a pair of muons ine+eμ+μXevents in the0.2129.000GeV/c2mass range, using178fb1of 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 ofXbeyond the standard model: aZvector boson in theLμLτmodel 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 theLμLτmodel and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below6GeV/c2, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for theLμLτmodel 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 Society2024 
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    Free, publicly-accessible full text available June 1, 2025