Measurement of $B^+\to\tau^+\nu_\tau$ branching fraction with a hadronic tagging method at Belle II

TL;DR

This paper reports the first evidence for the decay $B^+ o\tau^+\nu_\tau$ at Belle II, measuring its branching fraction using a hadronic tagging method with a large data sample.

Contribution

It provides the first evidence for $B^+\to\tau^+\nu_\tau$ decays at Belle II using hadronic tagging and a large dataset, measuring its branching fraction.

Findings

Evidence for $B^+\to\tau^+\nu_\tau$ at 3.0 sigma significance.

Measured branching fraction: $(1.24 \pm 0.41 \text{(stat.)} \pm 0.19 \text{(syst.)}) \times 10^{-4}$.

Large data sample of $387\times 10^6$ $\Upsilon(4S)$ events.

Abstract

We present a measurement of the branching fraction of $B^+\to\tau^+\nu_\tau$ decays using $(387\pm6)\times 10^6$ $\Upsilon(4S)$ 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 $B^+\to\tau^+\nu_\tau$ candidates are identified in the recoil. We find evidence for $B^+\to\tau^+\nu_\tau$ decays at 3.0 standard deviations, including systematic uncertainties. The measured branching fraction is $\mathcal{B}(B^+\to\tau^+\nu_\tau) = [1.24 \pm 0.41 (\text{stat.}) \pm 0.19 (\text{syst.})] \times 10^{-4}$.