We have studied the production of $B$ mesons in $e^{+}{e}^{-}$ annihilations and present new measurements of the semileptonic branching fractions for $B$-meson decay. We find $B(B\to e\nu X)=0.127\mathrm{}\pm 0.017\pm 0.013$ and $B(B\to \mu \nu X)=0.122\mathrm{}\pm 0.017\pm 0.031$, where the errors are statistical and systematic, respectively. The observed lepton momentum spectra from $B$ decays are presented. From these spectra we find the mean hadronic mass ($M_X$) recoiling against the detected lepton to be $M_X=2.2\mathrm{}\pm 0.2$ GeV/${\mathit{c}}^2$. We have also observed an enhancement in the lepton yield from the continuum above $B\overline{B}$ threshold. This enhancement is interpreted as continuum $B$ production and is consistent with what is expected from the naive quark model.

• Received 9 August 1982

DOI:https://doi.org/10.1103/PhysRevD.27.475