Measurement of the Inclusive Charmless Semileptonic Branching Ratio of B Mesons and Determination of |V_ub|

Abstract

We report a measurement of the inclusive charmless semileptonic branching fraction of B mesons in a sample of 89 million BBbar events recorded with the BABAR detector at the Y(4S) resonance. Events are selected by fully reconstructing the decay of one B meson and identifying a charged lepton from the decay of the other B meson. The number of signal events is extracted from the hadronic mass distribution and is used to determine the ratio of branching fractions BR(B->Xulnu)/BR(B->Xlnu) = (2.06 +/-0.25(stat) +/-0.23(syst) +/-0.36(theo)) x 10^{-2}. Using the measured branching fraction for inclusive semileptonic B decays, we find BR(B->Xulnu) = (2.24 +/- 0.27(stat) +/- 0.26(syst) +/- 0.39(theo)) x 10^{-3} and derive the CKM matrix element |Vub| = (4.62 +/- 0.28(stat) +/- 0.27(syst) +/- 0.48 (theo)) x 10^{-3}.