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First Measurement of

R (X_{τ / ℓ})

as an Inclusive Test of the

b \to c τ ν

Anomaly

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

Adachi, I; Adamczyk, K; Aggarwal, L; Ahmed, H; Aihara, H; Akopov, N; Aloisio, A; Asner, D M; Atmacan, H; Aushev, T; et al (May 2024, Physical Review Letters)

We measure the tau-to-light-lepton ratio of inclusive

B

-meson branching fractions

R (X_{τ / ℓ}) \equiv B (B \to X τ ν) / B (B \to X ℓ ν)

, where

ℓ

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 {fb}^{- 1}

of electron-positron collision data collected with the Belle II detector. We find

R (X_{τ / ℓ}) = 0.228 \pm 0.016 (stat) \pm 0.036 (syst)

, in agreement with standard-model expectations. This is the first direct measurement of

R (X_{τ / ℓ})

. Published by the American Physical Society2024

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