Measurement of $B^{+}\to {\tau }^{+}{\nu }_{\tau }$ branching fraction with a hadronic tagging method at Belle II

We present a measurement of the branching fraction of $B^{+}\to {\tau }^{+}{\nu }_{\tau }$ decays using $(387\pm 6)\times {10}^6$ $\mathrm{{\rm Y}}(4S)$ 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 $\mathcal{B}(B^{+}\to {\tau }^{+}{\nu }_{\tau })=[1.24\pm 0.41(\text{stat.})\pm 0.19(\text{syst.})]\times {10}^{-4}$.