Study of B̅ → X_u ℓν̅ decays in BB̅ events tagged by a fully reconstructed B-meson decay and determination of |V_{ub}|

We report measurements of partial branching fractions for inclusive charmless semileptonic B decays B̄→X ulν̄ and the determination of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |V ub|. The analysis is based on a sample of 467×106 Υ(4S)→BB̄ decays recorded with the BABAR detector at the PEP-II e +e - storage rings. We select events in which the decay of one of the B mesons is fully reconstructed and an electron or a muon signals the semileptonic decay of the other B meson. We measure partial branching fractions ΔB in several restricted regions of phase space and determine the CKM element |V ub| based on different QCD predictions. For decays with a charged lepton momentum pl*>1.0GeV in the B meson rest frame, we obtain ΔB=(1.80±0.13 stat±0.15 sys±0.02 theo)×10 -3 from a fit to the two-dimensional M X-q2 distribution. Here, M X refers to the invariant mass of the final state hadron X and q2 is the invariant mass squared of the charged lepton and neutrino. From this measurement we extract |V ub|=(4.33±0.24 exp±0.15 theo) ×10 -3 as the arithmetic average of four results obtained from four different QCD predictions of the partial rate. We separately determine partial branching fractions for B ̄0 and B - decays and derive a limit on the isospin breaking in B̄→X ulν̄ decays. .