Factorization for power corrections to B→Xsγ and B→Xuℓν¯

Abstract We derive factorization theorems for Λ QCD / m b power corrections to inclusive B-decays in the endpoint region, where m X 2 ∼ m b Λ QCD . In B → X u l ν ¯ our results are for the full triply differential rate. A complete enumeration of Λ QCD / m b corrections is given. We point out the presence of new Λ QCD / m b -suppressed shape functions, which arise at tree level with a 4π-enhanced coefficient, and show that these previously neglected terms induce an additional significant uncertainty for current inclusive methods of measuring | V u b | that depend on the endpoint region of phase space.