We report a measurement of the ratios of branching fractions R(D) = B(B¯ → Dτ−ν¯τ)/B(B¯ → D`−ν¯`) and R(D∗) = B(B¯ → D∗τ−ν¯τ)/B(B¯ → D∗`−ν¯`), where ` denotes an electron or a muon. The results are based on a data sample containing 772 × 106 BB¯ events recorded at the Υ(4S) resonance with the Belle detector at the KEKB e+e− collider. The analysis utilizes a method where 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 R(D) = 0.307 ± 0.037 ± 0.016 and R(D∗) = 0.283 ± 0.018 ± 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 and 1.1 standard deviations, respectively, while their combination agrees with the Standard Model predictions within 1.2 standard deviations.
|Publication status||Published - Apr 18 2019|
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