Non-factorizable effects in the B-(B)over-bar mixing

We analyse the corrections to factorization for the B-B mixing amplitude as suming that non-factorizable soft gluon exchanges can be described by the l ocal gluon condensate. Within this approximation the (alpha (s)GG)-correcti on to the mixing amplitude can be expressed through the B-meson transition form factors at zero momentum transfer. The correction is negative independ ent of the particular values of the form factors. As a next step, we study these form factors making use of the relativistic dispersion approach based on the constituent quark picture. We obtain spect ral representations for the form factors in terms of the B-meson soft wave function and the propagator of the color-octet by system. The B-meson wave function has been determined previously from weak exclusive B decays, so th e main uncertainty in our estimates comes from the unknown details of the p ropagator in the non-perturbative region. We obtain DeltaB(Bd)(m(b)) = -0.0 6 +/- 0.035 and DeltaB(Bs)(m(b)) = -0.05 +/- 0.03. (C) 2000 Elsevier Scienc e B.V. All rights reserved.