Measurement of the branching ratio of $\barB^0 \rightarrow D^*+ \tau^- \bar\nu_\tau$ relative to $\barB^0 \rightarrow D^*+ \ell^- \bar\nu_\ell$ decays with a semileptonic tagging method

摘要: We report a measurement of ratio R(D∗)=B(B¯0→D∗+τ−ν¯τ)/B(B¯0→D∗+ℓ−ν¯ℓ), where ℓ denotes an electron or a muon. The results are based on a data sample containing 772×106 BB¯ pairs recorded at the Υ(4S) resonance with the Belle detector at the KEKB e+e− collider. We select a sample of B0B¯0 pairs by reconstructing both B mesons in semileptonic decays to D∗∓ℓ±. We measure R(D∗)=0.302±0.030(stat)±0.011(syst), which is within 1.6σ of the Standard Model theoretical expectation, where σ is the standard deviation including systematic uncertainties.