Search for the \boldsymbolB⁰ → K⁰_{\rm S} τ⁺ τ^- decay

2026/07/28 by Belle, Belle II Collaborations, : +432
#hep-ex

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Abstract

We present the first search for B⁰ → K⁰_{\rm S} τ⁺τ^- decays. We look for signal decays in B⁰ B⁰ events produced in asymmetric-energy electron-positron collisions. This work uses samples from the Belle and Belle~II detectors, comprising 1.16 billion Υ(4S) events. In Υ(4S)→ B⁰B⁰ decays, the non-signal B⁰ meson is fully reconstructed in a hadronic channel. For the signal B⁰ meson, τ-lepton decays into final states with a single charged particle are selected. A multivariate classifier is used to combine several discriminating inputs into a single fit observable. We observe no evidence for the signal and set an upper limit on the branching fraction B(B⁰→ K⁰_{\rm S} τ⁺τ^-) < 8.3 × 10^-4 at the 90% confidence level. Combining this with the recent measurement of the isospin-partner decay B⁺→ K⁺τ⁺τ^-, we determine an upper limit B(B→ Kτ⁺τ^-) < 5.4×10^-4 at the 90% confidence level.

Citations

• The Imaging Time-of-Propagation Detector at Belle II
• Search for B⁰ → K^{∗ 0} τ⁺ τ^- decays at the Belle II experiment
• Uncertainty components in profile likelihood fits
• flavio: a Python package for flavour and precision phenomenology in the Standard Model and beyond
• Physics Achievements from the Belle Experiment
• Presentation of search results: the CL s technique
• The precision Monte Carlo event generator for two-fermion final states in collisions

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