Measurement of the branching ratio of B0 →d∗+τ- ν τ relative to B 0 →d∗+ℓ −νℓ decays with a semileptonic tagging method

Abstract

We report a measurement of the ratio R(D∗)=B(B0→D∗+τ-ντ)/B(B0→D∗+ν), where denotes an electron or a muon. The results are based on a data sample containing 772×106 BB pairs recorded at the (4S) 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∗ ±. We measure R(D∗)=0.302±0.030(stat)±0.011(syst), which is within 1.6σ 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.