Measurement of the $B^+ \to \eta\ell^+\nu$ and $B^+ \to \eta'\ell^+ \nu$ Branching Fractions using $\Upsilon(4S) \to B\bar{B}$ Events Tagged by a Fully Reconstructed B Meson

We report preliminary measurements of the exclusive charmless semileptonic branching fractions of the B+ --> eta l+ nu and B+ --> eta' l+ nu decays. These measurements are based on 316 fb-1 of data collected at the Y(4S) resonance by the BABAR detector. In events in which the decay of one B meson to a hadronic final state is fully reconstructed, the semileptonic decay of the recoiling B meson is identified by the detection of a charged lepton and an eta or eta'. We measure the branching fraction BR(B+ --> eta l+ nu) = (0.84 +/- 0.27 +/- 0.21) x 10-4, where the first error is statistical and the second one systematic. We also set an upper limit on the branching fraction of BR(B+ --> eta l+ nu) eta' l+ nu) < 1.3 x 10^-4 at the 90% confidence level.