Measurement of R(D) and R(D*) with a Semileptonic Tagging Method

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The experimental results on the ratios of branching fractionswhere l denotes an electron or a muon, show a long-standing discrepancy with the standard model predictions, and might hint at a violation of lepton flavor universality.We report a new simultaneous measurement of RðDÞ and RðD Ã Þ, based on a data sample containing 772 × 10 6 B B events recorded at the ϒð4SÞ resonance with the Belle detector at the KEKB e þ e -collider.In this analysis the tag-side B meson is reconstructed in a semileptonic decay mode and the signal-side τ is reconstructed in a purely leptonic decay.The measured values are RðDÞ ¼ 0.307 Æ 0.037 Æ 0.016 and RðD Ã Þ ¼ 0.283 Æ 0.018 Æ 0.014, where the first uncertainties are statistical and the second are systematic.These results are in agreement with the standard model predictions within 0.2, 1.1, and 0.8 standard deviations for RðDÞ, RðD Ã Þ, and their combination, respectively.This work constitutes the most precise measurements of RðDÞ and RðD Ã Þ performed to date as well as the first result for RðDÞ based on a semileptonic tagging method.

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