First measurement of $R(X_{τ/ell})$ as an inclusive test of the $b to c τν$ anomaly

Kavli Affiliate: T. Higuchi

| First 5 Authors: Belle II Collaboration, I. Adachi, K. Adamczyk, L. Aggarwal, H. Ahmed

| Summary:

We measure the tau-to-light-lepton ratio of inclusive $B$-meson branching
fractions $R(X_{tau/ell}) equiv mathcal{B}(Bto X tau nu)/mathcal{B}(B
to X ell nu)$, 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 $R(X_{tau/ell})$.

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