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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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 -
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.