An enhancement in the inclusive cross section for single muons produced in $e^{+}{e}^{-}$ annihilation at the $\Upsilon \mathrm{}\left(4S\right)\mathrm{}$ is observed, confirming the interpretation that a new bare flavor ($B$ mesons) is produced at the $\Upsilon \mathrm{}\left(4S\right)\mathrm{}$. A branching ratio for $B\to X\mu \nu$ of (9.4±3.6)% is obtained. The two-muon decay, $B\to X{\mu }^{+}{\mu }^{-}$ is not observed, providing a 90%-confidence-level upper limit for the branching ratio for that decay of 1.7%. Combining this with our previously reported limit of 5% for $B\to X{e}^{+}{e}^{-}$, we obtain 1.3% as an upper limit for $B\to X{l}^{+}{l}^{-}$.

• Received 31 October 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.88