$\mathcal{B}(B\rightarrow\tau\nu)$ measurement with the hadronic FEI at Belle~II

TL;DR

This paper reports a measurement of the $B\to\tau\nu$ decay branching fraction using Belle II data, providing evidence at 3.0 sigma and contributing to understanding of B meson decays and potential new physics.

Contribution

First measurement of $B\to\tau\nu$ at Belle II with hadronic tagging, achieving evidence at 3 sigma and refining the branching fraction estimate.

Findings

Evidence for $B\to\tau\nu$ at 3.0 sigma

Measured branching fraction: (1.24 ± 0.41 (stat) ± 0.19 (syst)) × 10^{-4}

Utilized hadronic tagging with Belle II data

Abstract

We presented at Moriond EW 2025 a measurement of the branching fraction of $B\to\tau\nu$ decays using $(387\pm 6) \times 10^6$ $\Upsilon (4S)$ collected between 2019 and 2022 with the Belle~II detector at the SuperKEKB $e^+e^-$ collider. We reconstruct the accompanying $B^-$ meson using the hadronic tagging method, while $B\to\tau\nu$ candidates are identified in the recoil. We find evidence for $B\to\tau\nu$ decays at 3.0 standard deviations, including systematic uncertainties. The measured branching fraction is $\mathcal{B}(B\rightarrow\tau\nu) = [1.24 \pm 0.41~\text{(stat.)} \pm 0.19~\text{(syst.)}] \times 10^{-4}$