decays and the extraction of |VERBAR;

The Cabibbo-Kobayashi-Maskawa matrix element vertical bar V-ub vertical bar is not well determined yet. It can be extracted from both inclusive or exclusive decays, like B -> pi(rho)l (nu) over bar (l). However, the exclusive determination from B -> rho l (nu) over bar (l), in particular, suffers from a large model dependence. In this paper, we propose to extract vertical bar V-ub vertical bar from the four-body semileptonic decay B -> pi pi l (nu) over bar (l), where the form factors for the pion-pion system are treated in dispersion theory. This is a model-independent approach that takes into account the pi pi rescattering effects, as well as the effect of the rho meson. We demonstrate that both finite-width effects of the rho meson as well as scalar pi pi contributions can be considered completely in this way.