Search for lepton-flavor-violating τ^- → ℓ^- K_s⁰ decays at Belle and Belle II

2025/04/22 by Belle, Belle Collaborations, Collaborations, Belle II +515
Computer Science · Decision Sciences · Physics and Astronomy · #Branching fraction #Collider #Computational Physics and Python Applications #Electron–positron annihilation #FOS: Physical sciences #High Energy Physics - Experiment (hep-ex) #KEKB #Particle identification #Particle physics theoretical and experimental studies #Scientific Computing and Data Management #Superconducting Super Collider

paper · open access · doi:10.48550/arxiv.2504.15745

published in arXiv (Cornell University) (Cornell University)

openalex publication_date 2025/04/22 · openalex created_date 2025/09/30 · openalex updated_date 2026/07/28

Abstract

We present the results of a search for charged-lepton-flavor violating decays τ^- → ℓ^-K_S⁰, where ℓ^- is either an electron or a muon. We combine e⁺e^- data samples recorded by the Belle II experiment at the SuperKEKB collider (428 fb^-1) with samples recorded by the Belle experiment at the KEKB collider (980 fb^-1) to obtain a sample of 1.3 billion e⁺e^-→τ⁺τ^- events. We observe 0 and 1 events and set 90% confidence level upper limits of 0.8 × 10^-8 and 1.2 × 10^-8 on the branching fractions of the decay modes τ^- → e^-K_S⁰ and τ^- → μ^-K_S⁰, respectively. These are the most stringent upper limits to date.

Citations

• Charged lepton flavor violation in light of the muon magnetic moment anomaly and colliders
• Lepton-flavour violation in hadronic tau decays and ℓ-τ conversion in nuclei
• Lepton-flavour violation in hadronic tau decays and μ-τ conversion in nuclei
• τ→ μμμ at a rate of one out of 10¹⁴ tau decays?
• Optuna: A Next-generation Hyperparameter Optimization Framework
• B2BII - Data conversion from Belle to Belle II
• The Belle II Core Software
• Flavor violating leptonic decays of τ and μ leptons in the Standard Model with massive neutrinos
• Front-end electronic readout system for the Belle II imaging Time-Of-Propagation detector
• XGBoost: A Scalable Tree Boosting System
• An Introduction to PYTHIA 8.2
• TREPS: A Monte-Carlo Event Generator for Two-photon Processes at e⁺e^- Colliders using an Equivalent Photon Approximation
• Lepton flavor-violating transitions in effective field theory and gluonic operators
• Physics Achievements from the Belle Experiment
• Constraints on two-lepton, two quark operators
• Photon pair production at flavour factories with per mille accuracy
• Tau and muon pair production cross-sections in electron-positron annihilations at sqrts = 10.58 GeV
• Matching perturbative and Parton Shower corrections to Bhabha process at flavour factories
• The BABAYAGA event generator
• Sensitivity of searches for new signals and its optimization
• Presentation of search results: theCLstechnique
• Electron identification in Belle
• An Improved Parton Shower algorithm in QED
• Review of Particle Physics
• Large-angle Bhabha scattering and luminosity at flavour factories
• The precision Monte Carlo event generator for two-fermion final states in collisions
• Confidence level computation for combining searches with small statistics
• Photos — a universal Monte Carlo for QED radiative corrections in decays

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• Search for the lepton-flavor-violating τ^- → e^∓ ℓ^± ℓ^∓ decays at Belle II
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• Search for lepton flavor violating τ→ ℓ V⁰ decays at Belle
• Search for lepton flavor-violating decay modes B⁰ → K^{∗ 0}τ^±ℓ^∓ (ℓ = e,μ) with hadronic B-tagging at Belle and Belle II
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