Inclusive and Exclusive Semileptonic Decays of B Mesons to D Mesons

Using the CLEO detector at the Cornell Electron Storage Ring (CESR), we have made the first measurements of the exclusive semileptonic branching fractions of B ^- mesons to the charmed final states D^0 and D^{ *0}:eqalign{ {rm B}(B^- to D^{ *0}l^-|nu)&= (4.1 +/- 0.8 _sp{-0.9}{+0.8 })({0.5over f_{+- }})%,cr{rm B }(B^- to D^0l^- |nu)&= (1.6 +/- 0.6 +/- 0.3)( {0.5over f_{+-}}) %.cr}We have confirmed the observation of the decay | B^0 to D^+l ^-|nu and have measured the branching fraction to be:{rm B}(| B^0 to D^+l ^-|nu) = (1.8 +/- 0.6 +/- 0.3)({0.5over f_{00} })%.These results yield the ratio of vector to pseudoscalar widths {Gamma _{SL}(D^ *)overGamma_{SL}(D)} = 2.6 _sp{-0.6}{+1.1} _sp{-0.8}{+1.0}. Further, we find the lifetime ratio for charged to neutral B mesons{tau(B^ -)overtau(B^0)} {rm to be} (0.89 +/- 0.19 +/- 0.13) ({f_{00}over f_ {+-}}).Using theoretical models, we extract a measurement of the KM matrix element | V_{cb}| = 0.040 +/- 0.006 +/- 0.003 +/- 0.004. We also find that the sum of the exclusive semileptonic branching fractions to pseudoscalar and vector meson final states only accounts for (64 +/- 10)% of the total semileptonic branching fraction to charmed final states. The bulk of this work is published in R. Fulton et al., Physical Review D, 1 February 1991.