An enhancement has been observed in the inclusive cross section for direct single electrons produced in $e^{+}{e}^{-}$ annihilations at the $\Upsilon \mathrm{}\left(4S\right)\mathrm{}$. This is interpreted as evidence for a new weakly decaying particle, the $B$ meson. A branching ratio for $B\to \mathrm{Xe}\nu$ of [13±3 (±3)]% is inferred, where the first set of errors is statistical and the estimated systematic error is enclosed in parentheses.

• Received 1 October 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.84