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

Abstract

We report a measurement of the 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 × 1 0 6

B ¯ B pairs recorded at the Υ ( 4 S ) resonance with the Belle detector at the KEKB e + e − collider. We select a sample of B 0 ¯ B 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 1.6 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.