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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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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 -
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. -
We measure the tau-to-light-lepton ratio of inclusive-meson branching fractions, whereindicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructedmeson and a charged lepton candidate fromof electron-positron collision data collected with the Belle II detector. We find, in agreement with standard-model expectations. This is the first direct measurement of.
Published by the American Physical Society 2024 Free, publicly-accessible full text available May 1, 2025