Instantons and the end point of the lepton energy spectrum in charmless semileptonicBdecays

A recent calculation by Chay and Rey has shown that instantons may make a significant contribution to the lepton energy spectrum near its end point. Using an ansatz borrowed from the study of high energy baryon-number-violating processes, we investigate whether these corrections could spoil the relation between the nonperturbative contributions to this spectrum and to the photon energy spectrum in radiative B decays. We find, in general, that this universality may well fail unless the spectrum is smeared over a region which is considerably larger than had previously been thought necessary. This result affects the possibility of performing a reliable measurement of ${\mathit{V}}_{\mathit{u}\mathit{b}}$ using inclusive decays.