QCD Anatomy of B -> X_s gamma Decays

Abstract

We present an updated next-to-leading order analysis of the B -> X_s gamma branching ratio and photon spectrum, including consistently the effects of Fermi motion in the heavy-quark expansion. For the Standard Model, we obtain B(B -> X_s gamma) = (2.57+-0.26^{+0.31}_{-0.36}) * 10^{-4} for the integral over the high-energy part of the photon spectrum with E_gamma^{lab} > 2.2 GeV, where the first error reflects the uncertainty in the input parameters, and the second one the uncertainty in the calculation of Fermi motion. This prediction agrees with the CLEO measurement of the same quantity within one standard deviation. From a reanalysis of the CLEO data, we obtain for the total branching ratio B(B -> X_s gamma) = (2.62+-0.60_{exp}^{+0.37}^{-0.30{th}}) * 10^{-4} using the measured rate above 2.2 GeV, and (2.66+-0.56_{exp}^{+0.43}_{-0.48{th}}) * 10^{-4} using a fit to the photon energy spectrum. Both values are consistent with the Standard Model prediction of (3.29+-0.33) * 10^{-4}. Our analysis contains an improved discussion of renormalization scale dependence and QED corrections. We also discuss the sensitivity of the branching ratio and photon spectrum to hadronic parameters such as the b-quark mass, and to possible contributions from New Physics beyond the Standard Model.