Measurement of the branching ratio of B¯0 → D∗ τ −ν¯τ relative to B¯0 → D∗ ℓ −ν¯ℓ decays with a semileptonic tagging method

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 B 0B¯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