Study of (B)over-bar -> X(u)l(v)over-bar decays in B(B)over-bar events tagged by a fully reconstructed B-meson decay and determination of vertical bar V-ub vertical bar

We report measurements of partial branching fractions for inclusive charmless semileptonic B decays B⎯⎯⎯→Xuℓν⎯⎯ and the determination of the Cabibbo–Kobayashi–Maskawa (CKM) matrix element |Vub|. The analysis is based on a sample of 467×106 Υ(4S)→BB⎯⎯⎯ 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 |Vub| based on different QCD predictions. For decays with a charged lepton momentum p∗ℓ>1.0  GeV in the B meson rest frame, we obtain Δℬ=(1.80±0.13stat±0.15sys±0.02theo)×10−3 from a fit to the two-dimensional MX−q2 distribution. Here, MX 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 |Vub|=(4.33±0.24exp±0.15theo)×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 B⎯⎯⎯→Xuℓν⎯⎯ decays.