Full Reconstruction of Semileptonic B Meson Decays.

Using data collected with the CLEO II detector at the Cornell Electron Storage Ring, we have succeeded in fully reconstructing semileptonic decays of B mesons. The hermeticity of the CLEO II detector and its excellent electromagnetic calorimeter allow inference of neutrino momenta in selected events. By combining these 'neutrinos' with other decay products of B mesons, we have reconstructed the decay chain B^{+/- } to D^0l^{+/-} nu, D^0 to K^-pi^+ . Using an independently measured value for {cal B}(B to D^{* }lnu), we measure the branching fraction {cal B}(B^+ to D^0l ^+| nu) to be 2.4 +/- 1.2% (where the error is statistical only). This is consistent with previous measurements and theoretical expectations. Applying the same technique to charmless decays of B mesons, we have searched for the decays B ^0topi^{+/-}lnu, B^{+/-} to pi ^0lnu, B^0torho^ {+/-}lnu, and B^ {+/-}torho^0lnu . We observe a total of 36 events consistent with these processes. We estimate that fewer than 14 of the events are due to backgrounds or misreconstruction. We infer branching ratios ofeqalign {{cal B}(B^0topi ^{+/-}lnu) &= (1.9{-}2.5 +/- 0.6 +/- 0.2) times 10^{ -4}cr{cal B}(B^0to rho^+/- lnu) &= (0.5{-}1.3 +/- 0.8 +/- 0.2) times 10^{ -4}cr}where the first errors are statistical and the second are systematic. The ranges of central values reflect dependence on the theoretical models employed. These observations are consistent with previously published upper limits, but the vector to pseudoscalar ratio is completely unexpected. The implications of this result are discussed.