Measuring the photon energy spectrum in radiative B decays provides essential help for gaining theoretical control over semileptonic B transitions. The hadronic recoil mass distribution in B → Xuℓ ν promises the best environment for determining |Vub|. The theoretical uncertainties are largest in the domain of low values of the lepton pair mass q2. Universality relations allow us to describe this domain reliably in terms of the photon spectrum in B → Xs+ γ. A method is proposed to incorporate 1/mbcorrections into this relation. The low-Eγtail in radiative decays is important in the context of extracting |Vub|. We argue that CLEO's recent fit to the spectrum underestimates the fraction of the photon spectrum below 27thinsp;GeV. Potentially significant uncertainties enter in the theoretical evaluation of the integrated end point lepton spectrum or the B → Xuℓ ν width with a too high value of the lower cut on q2in alternative approaches to |Vub|.
ON THE EXPECTED PHOTON SPECTRUM IN B → Xs+ γ AND ITS USES
