Electroweak effects in the B-0-(B)over-bar(0) mixing

We compute analytically the complete electroweak two-loop corrections to th e B-0-(B) over bar(0) mixing. These corrections fix the normalization of th e electroweak coupling employed in the extraction of \V-td\ and reduce the theoretical uncertainty due to higher order electroweak effects from severa l percent to a few parts in a thousand. If the LO result is expressed in te rms of G(mu) or of the <(MS)over bar> coupling (g) over cap(M-Z), the two-l oop corrections are O(1%), the exact value depending on the mass of the Hig gs boson. We discuss in detail the renormalization procedure and the scheme and scale dependence, and provide practical formulas for the numerical imp lementation of our results. We also consider the heavy top mass expansion a nd show that in the case at hand it converges very slowly. (C) 1999 Elsevie r Science B.V.