First Measurement of the Ratio<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">B</mml:mi><mml:mo>(</mml:mo><mml:mi mathvariant="italic">t</mml:mi><mml:mi/><mml:mo>→</mml:mo><mml:mi/><mml:mi mathvariant="italic">Wb</mml:mi><mml:mo>)</mml:mo><mml:mi>/</mml:mi><mml:mi mathvariant="italic">B</mml:mi><mml:mo>(</mml:mo><mml:mi mathvariant="italic">t</mml:mi><mml:mi/><mml:mo>→</mml:mo><mml:mi/><mml:mi mathvariant="italic">Wq</mml:mi><mml:mo>)</mml:mo></mml:math>and Associated Limit on the Cabibbo-Kobayashi-Maskawa Element<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>|</mml:mi><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="italic">V</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">tb</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mi>|</mml:mi></mml:math>

We present the first measurement of the ratio of branching fractions $R\ensuremath{\equiv}B(t\ensuremath{\rightarrow}\mathrm{Wb})/B(t\ensuremath{\rightarrow}\mathrm{Wq})$ from $p\overline{p}$ collisions at $\sqrt{s}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.8\mathrm{TeV}$. The data set corresponds to $109{\mathrm{pb}}^{\ensuremath{-}1}$ of data recorded by the Collider Detector at Fermilab during the 1992--95 Tevatron run. We measure ${R\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.94}_{\ensuremath{-}0.24}^{+0.31}(\mathrm{stat}+\mathrm{syst})$ or $R>0.61$ (0.56) at $90%$ $(95)%$ C.L., in agreement with the standard model predictions. This measurement yields a limit on the Cabibbo-Kobayashi-Maskawa quark mixing matrix element $|{V}_{\mathrm{tb}}|$ under the assumption of three generations and unitarity.

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