Measurement of the B+ → ω ℓ⁺ 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) x 10⁻⁴, where the uncertainties are statistical and systematic, respectively. We also present measurements of the partial branching fractions in three bins of q², the invariant-mass squared of the lepton-neutrino system, and we compare them to theoretical predictions of the form factors.