Determining V(ub) from B+ --> D*+(s) e+ e- and B+ --> D*+ e+ e-

Abstract

It was recently pointed out that the decays B^+ --> D^{*+}_s gamma and B^+ --> D^{*+} gamma can be used for an extraction of |V_{ub}|. The theory of these decays is poorly understood. It was shown that in a world of almost degenerate b and c-quarks the decay would be computable. The severe difficulties that are encountered in the realistic calculation stem primarily from the very hard photon produced in the two body decay. We point out that in the decays B^+ --> D^{*+}_s e^+e^- and B^+ --> D^{*+} e^+e^- the photon vertex is soft when the charmed meson is nearly at rest (in the B^+ rest frame). This allows us to compute with some confidence the decay rate in a restricted but interesting kinematic regime. Given enough data the extraction of V_{ub} with reasonably small uncertainties could proceed through an analysis of these exclusive decays much as is done in the determination of V_{cb}.