Quark mass ratios in semi-leptonic quark decays

An ambiguity in the calculation of CKM matrix elements from semi- leptonic decay rates is resolved: to every choice of scales for the quark masses in the phase-space factor and in the QCD-correction factor, there corresponds a specific QCD-correction factor. This factor is modified in such a way as to make the final result independent of the scales. Specific expressions are given for the case of on-shell quark masses and for the case where both masses are taken at a single common scale. A calculation of V/sub cb/ and V/sub ub//V/sub cb/ is carried out in view of these clarifications. 5 refs