Please take a look at the file "Feynman.pdf". The ratio rate for the two decays is shown in the second file, "rate.pdf". I understand it takes that form because the usW vertex is Carribbo supressed relative to the udW vertex. My concern is that the ratio is only squared. Rate is proportional to the amplitude *squared*. And the amplitude is proportional to the square of the coupling constant (multiplied by the propagator). So the transition rate must be proportional to the fourth power of the coupling constant, right? Is the expression in rate.pdf wrong?