As it is well known, large threshold logarithmic contributions can be resummed in the form factors within a fixed order approximation. Such form factors, however, are divergent at large x, as a consequence of the QCD coupling hitting the Landau pole in the resumming procedure. A possible solution is presented here, and it requires using an effective coupling which takes into account additional –usually neglected– absorptive physical effects; as a consequence, the theory becomes completely predictive with regular form factors at large x. These results are pertinent to a wide class of phenomenological applications, and in particular are of interest for processes at the B meson mass scale.
Measurement of the lifetimes of $B$ meson mass Eigenstates