Study of B̅ →Xuℓν̅ decays in BB̅ events tagged by a fully reconstructed B-meson decay and determination of |V[subscript ub]|

Abstract

We report measurements of partial branching fractions for inclusive charmless semileptonic B decays B̅ →Xuℓν̅ and the determination of the Cabibbo–Kobayashi–Maskawa (CKM) matrix element |Vub|. The analysis is based on a sample of 467×10[superscript 6] Υ(4S)→BB̅ decays recorded with the BABAR detector at the PEP-II e[superscript +]e[superscript -] 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[subscript ub]| based on different QCD predictions. For decays with a charged lepton momentum p[subscript ℓ][superscript *]>1.0 GeV in the B meson rest frame, we obtain ΔB=(1.80±0.13[subscript stat]±0.15[subscript sys]±0.02[subscript theo])×10[superscript -3] from a fit to the two-dimensional M[subscript X]-q[superscript 2] distribution. Here, M[subscript X] refers to the invariant mass of the final state hadron X and q[superscript 2] is the invariant mass squared of the charged lepton and neutrino. From this measurement we extract |V[subscript ub]|=(4.33±0.24[subscript exp]±0.15[subscript theo])×10[superscript -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 B̅[superscript 0] and B[superscript -] decays and derive a limit on the isospin breaking in B̅→X[subscript u]ℓν̅ decays.