ONE-LOOP RADIATIVE-CORRECTIONS TO A HEAVY TOP DECAY IN THE STANDARD MODEL

We study one-loop radiative corrections to the top-quark decay: t --> bW. For a heavy top (i.e. m(t) greater-than-or-similar-to 100 GeV), simple analytic formulas are derived. For the inclusive process, t --> bW + X due to QCD, cancellations of infrared and mass singularities are shown explicity. The final result agrees with an earlier calculation in the literature, and is an 8% reduction in the partial width for alpha-s = 0.1. Also, leading terms of the electroweak corrections are shown to be proportional to an enhancement factor m(t)2/M(W)2. Their effect is about 1-3%. We also study the angular distributions of the leptons from the subsequent exclusive (or semi-inclusive) W decay. An asymmetry parameter which measures the polarizations of the W is introduced. We find that it is sensitive to the QCD corrections especially in the vicinity of m(t) = square-root 2M(W).