We report a measurement of the branching fraction ratios R(D ( * ) ) ofB → D ( * ) τ −ν τ relative tō B → D ( * ) −ν (where = e or µ) using the full Belle data sample of 772 × 10 6 BB pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e + e − collider. The measured values are R(D) = 0.375 ± 0.064(stat.) ± 0.026(syst.) and R(D * ) = 0.293 ± 0.038(stat.) ± 0.015(syst.). The analysis uses hadronic reconstruction of the tag-side B meson and purely leptonic τ decays. The results are consistent with earlier measurements and do not show a significant deviation from the standard model prediction.
We report the first measurement of the τ lepton polarization P_{τ}(D^{*}) in the decay B[over ¯]→D^{*}τ^{-}ν[over ¯]_{τ} as well as a new measurement of the ratio of the branching fractions R(D^{*})=B(B[over ¯]→D^{*}τ^{-}ν[over ¯]_{τ})/B(B[over ¯]→D^{*}ℓ^{-}ν[over ¯]_{ℓ}), where ℓ^{-} denotes an electron or a muon, and the τ is reconstructed in the modes τ^{-}→π^{-}ν_{τ} and τ^{-}→ρ^{-}ν_{τ}. We use the full data sample of 772×10^{6} BB[over ¯] pairs recorded with the Belle detector at the KEKB electron-positron collider. Our results, P_{τ}(D^{*})=-0.38±0.51(stat)_{-0.16}^{+0.21}(syst) and R(D^{*})=0.270±0.035(stat)_{-0.025}^{+0.028}(syst), are consistent with the theoretical predictions of the standard model.
We report the results of a study of the exclusive semileptonic decays B − → π 0 ℓ −ν ℓ ,B 0 → π + ℓ −ν ℓ , B − → ρ 0 ℓ −ν ℓ ,B 0 → ρ + ℓ −ν ℓ and B − → ωℓ −ν ℓ , where ℓ represents an electron or a muon. The events are tagged by fully reconstructing a second B meson in the event in a hadronic decay mode. The measured branching fractions are B(B − → π 0 ℓ −ν ℓ) = (0.80 ± 0.08 ± 0.04) × 10 −4 , B(B 0 → π + ℓ −ν ℓ) = (1.49 ± 0.09 ± 0.07) × 10 −4 , B(B − → ρ 0 ℓ −ν ℓ) = (1.83 ± 0.10 ± 0.10) × 10 −4 , B(B 0 → ρ + ℓ −ν ℓ) = (3.22 ± 0.27 ± 0.24) × 10 −4 , and B(B − → ωℓ −ν ℓ) = (1.07 ± 0.16 ± 0.07) × 10 −4 , where the first error is statistical and the second one is systematic. The obtained branching fractions are inclusive of soft photon emission. We also determine the branching fractions as a function of the 4-momentum transfer squared to the leptonic system q 2 = (p ℓ + p ν) 2 , where p ℓ and p ν are the lepton and neutrino 4-momenta, respectively. Using the pion modes, a recent LCSR calculation, lattice QCD results and a model-independent description of the hadronic form factor, a value of the CKM matrix element |V ub | = (3.52 ± 0.29) × 10 −3 is extracted. For the first time, a charmless state with invariant mass greater than 1 GeV/c 2 , which might be dominated by the decay B − → f 2 ℓ −ν ℓ , is observed. These results are obtained from a 711 fb −1 data sample that contains 772 × 10 6 BB pairs, collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e + e − collider.
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