Evidence for New-Flavor Production at the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>ϒ</mml:mi><mml:mn/><mml:mo>(</mml:mo><mml:mn>4</mml:mn><mml:mi>S</mml:mi><mml:mo>)</mml:mo><mml:mn/></mml:math>

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