Measurement of the branching ratio of B0→D*+⁢τ−⁢ν_τ relative to B0→D*+⁢ℓ−ν_ℓ decays with a semileptonic tagging method

We report a measurement of the ratio R⁡(D*)=B⁡(B0→D*+⁢τ−⁢ν_τ)/B⁡(B0→D*+⁢ℓ−⁢ν_ℓ), where ℓ denotes an electron or a muon. The results are based on a data sample containing 772×1⁢0^6  BB pairs recorded at the Y(4⁢S) resonance with the Belle detector at the KEKB e+e− collider. We select a sample of B0B0 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 1.6⁢σ of the Standard Model theoretical expectation, where the standard deviation σ includes systematic uncertainties. We use this measurement to constrain several scenarios of new physics in a model-independent approach.