We have searched for the decay $B^{+}\to \omega l^{+}\nu$ ($l=e$ or $\mu$) in $78{\mathrm{f}\mathrm{b}}^{-1}$ of ${\rm Y}(4S)$ data ($85\times {10}^6$$B\overline{B}$ events) accumulated with the Belle detector. The final state is fully reconstructed using the $\omega$ decay into ${\pi }^{+}{\pi }^{-}{\pi }^0$, combined with detector hermeticity to estimate the neutrino momentum. A signal of $414\pm 125$ events is found in the data, corresponding to a branching fraction of $(1.3\pm 0.4\pm 0.2\pm 0.3)\times {10}^{-4}$, where the first two errors are statistical and systematic, respectively. The third error reflects the estimated form-factor uncertainty.

• Received 12 February 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.131803