Measurement of the $B^{+}\to \omega {\ell }^{+}\nu$ branching fraction with semileptonically tagged $B$ mesons

We report a measurement of the branching fraction of the exclusive charmless semileptonic decay $B^{+}\to \omega {\ell }^{+}\nu$, where $\ell$ 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 $426.1{\mathrm{fb}}^{-1}$ 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 $\mathcal{B}(B^{+}\to \omega {\ell }^{+}\nu )=(1.35\pm 0.21\pm 0.11)\times {10}^{-4}$, 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.

Figure 1

Event kinematics of a decay $B_{\mathrm{sig}}\to X\nu$, $B_{\mathrm{tag}}\to Y\nu$. The various angles and momenta are described in the text.

Figure 2

Distribution of ${cos}^2{\varphi }_B$. Points represent data, the curves are stacked fit results for “continuum background” (dashed), “$B\overline{B}$ background” (dotted), and “signal” (solid) PDFs, as defined in the text.

Figure 3

The partial branching fractions $\Delta \mathcal{B}(B^{+}\to \omega {\ell }^{+}\nu )/\Delta q^2$ in bins of $q^2$. The points are the measurements, with the error bars indicating the statistical uncertainty, and the quadratic sum of statistical and systematic uncertainties, respectively. The histograms are predictions by Ball-Zwicky (dotted blue), and ISGW2 (dashed green) calculations, each scaled to the measured total branching fraction.