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We report a search for a heavy neutral lepton (HNL) that mixes predominantly with${\nu }_{\tau }$. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy$e^{+}{e}^{-}$collider. The data sample was collected at and just below the center-of-mass energies of the$\mathrm{\Upsilon }(4S)$and$\mathrm{\Upsilon }(5S)$resonances and has an integrated luminosity of$915{\mathrm{fb}}^{-1}$, corresponding to$(836\pm 12)\times {10}^6$$e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$events. We search for production of the HNL (denoted$N$) in the decay${\tau }^{-}\to {\pi }^{-}N$followed by its decay via$N\to {\mu }^{+}{\mu }^{-}{\nu }_{\tau }$. The search focuses on the parameter-space region in which the HNL is long-lived, so that the${\mu }^{+}{\mu }^{-}$originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the$\tau$neutrino as a function of the HNL mass.

Published by the American Physical Society2024 

We report a search for the charged-lepton flavor violation in Υ(2S) →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 ($$ \mathcal{B} $$$B$) at 90% confidence level. We obtain the most stringent upper limits:$$ \mathcal{B} $$$B$(Υ(2S)→ μ^∓τ^±)<0.23×10⁻⁶and$$ \mathcal{B} $$$B$(Υ(2S)→ e^∓τ^±)<1.12×10⁻⁶.

 

We present the complete cross-section for the production of unpolarized hadrons in semi-inclusive deep-inelastic scattering up to power-suppressed O(1/Q^2) terms in the Wandzura-Wilczek-type approximation, which consists in systematically assuming that qgqbar-terms are much smaller than qqbar -correlators. We compute all twist-2 and twist-3 structure functions and the corresponding asymmetries, and discuss the applicability of the Wandzura-Wilczek-type approximations on the basis of available data. We make predictions that can be tested by data from COMPASS, HERMES, Jefferson Lab, and the future Electron-Ion Collider. The results of this paper can be readily used for phenomenology and for event generators, and will help to improve the description of semi-inclusive deep-inelastic processes in terms of transverse momentum dependent parton distribution functions and fragmentation functions beyond the leading twist.