Study of exclusive B→X_uℓν decays and extraction of |V_ub| using full reconstruction tagging at the Belle experiment

We report the results of a study of the exclusive semileptonic decays B^-→π0ℓ^-ν_̄ℓ, B ^̄0→π⁺ℓ ^-ν_̄ℓ, B^-→ρ0ℓ ^-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^-→π0ℓ ^-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^-→ρ0ℓ ^-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 q2=(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 light cone sum rule calculation, lattice QCD results and a model-independent description of the hadronic form factor, a value of the Cabibbo-Kobayashi- Maskawa matrix element |V_ub|=(3.52±0.29)×10^-3 is extracted. A structure in the two-pion invariant mass distribution near 1.3 GeV/c2, which might be dominated by the decay B^-→f ₂(1270)ℓ^-ν_̄ℓ, f ₂→π⁺π^-, is seen. These results are obtained from a 711 fb^-1 data sample that contains 772×106 BB̄ pairs, collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider.