Some Theoretical Results on V Ub and V Cb

It was proposed [1] recently to use the decay distribution in terms of the observable ξu = (q 0 + |q|)/MB in the B rest frame to measure Vub, where q is the momentum transfer to the lepton pair. This decay spectrum is unique in that the tree-level and virtual gluon processes b → ulν at the parton level generate a trivial ξu spectrum — a discrete line at ξu = mb/MB, solely on kinematic grounds. Two distinct effects, gluon bremsstrahlung and hadronic bound state effects, spread out the spectrum, but most of the decay rate remains at large ξu. Consequently, about 99% of the b → u events pass the kinematic cut ξu > 1 − MD/MB, where no b → c transition is allowed. This discrimination between b → u signal and b → c background is even more efficient than the cut on the hadronic invariant mass.