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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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  4. 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
  5. A<sc>bstract</sc>

    We measure the cross section ofe+eηcJ/ψat 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 fb1. We also search for double charmonium production ine+eηcJ/ψvia initial state radiation near theηcJ/ψthreshold. No evident signal of the double charmonium state is found, but evidence for thee+e→ ηcJ/ψprocess is found with a statistical significance greater than 3.3σnear theηcJ/ψthreshold. The average cross section near the threshold is measured and upper limits of cross sections are set for other regions.

     
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