First Measurement of the B -> pi lepton neutrino And B -> rho(omega) lepton neutrino Branching Fractions

CLEO has studied B decays to the final states {pi}{ell}{nu}, {rho}{ell}{nu}, and {omega}{ell}{nu}, where {ell} = e or {mu}. We fully reconstruct these modes using a measurement of the missing energy and momentum in each event to infer the neutrino momentum. With the B{sup 0} and B{sup +} modes combined according to isospin predictions for the relative partial widths, we obtain {Beta}(B{sup 0} {yields} {pi}{sup -}{ell}{sup +}{nu}) = (1.8 {+-} 0.4 {+-} 0.3 {+-} 0.2) x 10{sup -4} and {Beta}(B{sup 0} {yields} {rho}{sup -}{ell}{sup +}{nu}) = (2.5 {+-} 0.4{sub 0.7}{sup -.5} {+-} 0.5) x 10{sup -4}, where the errors are statistical, systematic and the estimated model-dependence. We also estimate |V{sub ub}| = (3.3 {+-} 0.2{sub 0.4}{sup +0.3} {+-} 0.7) x 10{sup -3}.