On The Expected Photon Spectrum in B -> X_s + gamma and Its Uses
Abstract
Description
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 -> X_u \ellνpromises the best environment for determining |V_ub|. The theoretical uncertainties are largest in the domain of low values of the lepton pair mass q^2. Universality relations allow to describe this domain reliably in terms of the photon spectrum in B -> X_s + γ. A method is proposed to incorporate 1/m_b corrections into this relation. The low-E_γtail in radiative decays is important in the context of extracting |V_ub|. 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 -> X_u \ellνwidth with a too high value of the lower cut on q^2 in alternative approaches to |V_ub|.
24 pages, 6 figures, LaTeX. Revised: Complete version. Numerical predictions are improved and the estimate for the decay fraction revised. The theoretical expectations for the decay fraction and the spectrum itself are given on the plots
24 pages, 6 figures, LaTeX. Revised: Complete version. Numerical predictions are improved and the estimate for the decay fraction revised. The theoretical expectations for the decay fraction and the spectrum itself are given on the plots