Transverse mass as a means of measuring the W width at the Tevatron

At the Tevatron the transverse mass is used to separate on mass shell from off mass shell W production; and the rate of off mass shell W production gives a measure of the W width. We look at alternative variables to see if the separation of on and off mass shell W's can be improved, and hence give a better measure of the W width. We find that the transverse mass is very close to the optimal variable for separating on from off mass shell W decay, and hence there is little to be gained by using other, more complicated, variables. This happens because if the transverse mass is above the W mass, the W is guaranteed to be produced off mass shell.