First Measurement of the Ratio $B (t \to Wb) / B (t \to Wq)$ and Associated Limit on the Cabibbo-Kobayashi-Maskawa Element $| V_{tb} |$

We present the first measurement of the ratio of branching fractions

R \equiv B (t \to Wb) / B (t \to Wq)

from

p \bar{p}

collisions at

\sqrt{s} = 1.8 TeV

. The data set corresponds to

109 {pb}^{- 1}

of data recorded by the Collider Detector at Fermilab during the 1992–95 Tevatron run. We measure

{R = 0.94}_{- 0.24}^{+ 0.31} (stat + syst)

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_{tb} |

under the assumption of three generations and unitarity.

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First Measurement of the RatioB(t→Wb)/B(t→Wq)and Associated Limit on the Cabibbo-Kobayashi-Maskawa Element|Vtb|