Measurement of the B^+ → ωℓ^+ν branching fraction with semileptonically tagged B mesons

We report a measurement of the branching fraction of the exclusive charmless semileptonic decay B^+→ωl^+ν, where l is either an electron or a muon. We use samples of B^+ mesons tagged by a reconstructed charmed semileptonic decay of the other B meson in the event. The measurement is based on a data set of 426.1  fb^(-1) of e^+e^- collisions at a center-of-mass energy of 10.58 GeV recorded with the BABAR detector at the PEP-II asymmetric-energy e^+e^- storage rings. We measure a branching fraction of B(B^+→ωl^+ν)=(1.35±0.21±0.11)×10^(-4), where the uncertainties are statistical and systematic, respectively. We also present measurements of the partial branching fractions in three bins of q^2, the invariant-mass squared of the lepton-neutrino system, and we compare them to theoretical predictions of the form factors.