We measure the tau-to-light-lepton ratio of inclusive $B$-meson branching fractions $R(X_{\tau /\ell })\equiv \mathcal{B}(B\to X\tau \nu )/\mathcal{B}(B\to X\ell \nu )$, where $\ell$ indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed $B$ meson and a charged lepton candidate from $189{\mathrm{fb}}^{-1}$ of electron-positron collision data collected with the Belle II detector. We find $R(X_{\tau /\ell })=0.228\pm 0.016(\mathrm{stat})\pm 0.036(\mathrm{syst})$, in agreement with standard-model expectations. This is the first direct measurement of $R(X_{\tau /\ell })$.

• Received 14 November 2023
• Accepted 19 April 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.211804

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Published by the American Physical Society