PRL 132, 211804 (2024)

SCIE

First Measurement of R(Xτ /ℓ) as an Inclusive Test of the b → cτ ν Anomaly

Belle II Collaboration (I. Adachi, ..., T.V. Dong, ...)

We measure the tau-to-light-lepton ratio of inclusive B-meson branching fractions R(Xτ/ℓ) ≡ B(B → Xτ ν)/B(B → Xℓν), 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(Xτ/ℓ) = 0.228 ± 0.016 (stat) ± 0.036 (syst), in agreement with standard-model expectations. This is the first direct measurement of R(Xτ/ℓ).

DOI: https://doi.org/10.48550/arXiv.2311.07248

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