On The Expected Photon Spectrum in B -> Xs + gamma and Its Uses
Abstract
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 -> Xu 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 to describe this domain reliably in terms of the photon spectrum in B -> Xs + γ. A method is proposed to incorporate 1/mb corrections into this relation. The low-Eγ tail 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 2 GeV. Potentially significant uncertainties enter in the theoretical evaluation of the integrated end-point lepton spectrum or the B -> Xu width with a too high value of the lower cut on q2 in alternative approaches to |Vub|.
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