Study of b --> ulnu Decays on the Recoil of Fully Reconstructed B Mesons and Determination of Vub

Abstract

Based on 88 million Y(4S)->BB decays collected by the BaBar experiment at the PEP-II asymmetric-energy B factory at SLAC, we report preliminary results of four analyses which study in semileptonic charmless B decays, B->Xulnu and measure Vub. In events in which one B meson decay to a hadronic final state is fully reconstructed, the semileptonic decay of the second B meson is identified by the detection of a charged lepton. From both the spectrum of the mass of the hadronic system (mX) and the two-dimensional distribution of mX and q^2 we derive the charmless semileptonic branching fraction and we extract Vub=(4.77\pm 0.28(stat.)\pm 0.25 (sys.)^{+0.65}_{-0.39}(theo.)) 10^-3 and Vub=(4.92\pm 0.39(stat.)\pm 0.36 (sys.)\pm 0.46(theo.)) 10^-3, respectively. We use the same sample to extract the true mX distribution for B->Xulnu events. Finally, we identify several exclusive charmless semileptonic B decays, and measure the branching fractions BR(B->pilnu) = (1.08 \pm 0.28(stat.) \pm 0.16(sys.))10^-4 and BR(B-> rho lnu) = (2.57 \pm 0.52(stat.) \pm 0.59(sys.)) 10^-4 using isospin and quark model constraints. We also set limits on BR(B^- -> eta l nu), BR(B^- -> eta' l nu), $BR(B^- -> a^0_0 l nu), and $BR(B^0 -> a^+_0lnu).