Measurement of $\mathcal{R}(D)$ and $\mathcal{R}(D^{\ast })$ with a semileptonic tagging method

Abstract

We report a measurement of the ratios of branching fractions $\mathcal{R}(D)=\mathcal{B}(\bar{B}\to D{\tau }^{-}{\bar{\nu }}_{\tau })/\mathcal{B}(\bar{B}\to D{\ell }^{-}{\bar{\nu }}_{\ell })$ and $\mathcal{R}(D^{\ast })=\mathcal{B}(\bar{B}\to D^{\ast }{\tau }^{-}{\bar{\nu }}_{\tau })/\mathcal{B}(\bar{B}\to D^{\ast }{\ell }^{-}{\bar{\nu }}_{\ell })$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times 10^6$ $B\bar{B}$ events recorded at the ${\rm Y}(4S)$ 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 $\tau$ is reconstructed in a purely leptonic decay. The measured values are $\mathcal{R}(D)=0.307\pm 0.037\pm 0.016$ and $\mathcal{R}(D^{\ast })=0.283\pm 0.018\pm 0.014$, 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.

12 pages, 6 figures

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