Factorization at subleading power and irreducible uncertainties in B̄ → X_sγ decay

Using methods from soft-collinear and heavy-quark effective theory, a systematic factorization analysis is performed for the B̄ → X _sγ photon spectrum in the endpoint region m_b-2E _γ = O(Λ_QCD). It is proposed that, to all orders in 1/m_b, the spectrum obeys a novel factorization formula, which besides terms with the structure H J⊗S familiar from inclusive B̄ → X_ulv̄ decay distributions contains "resolved photon" contributions of the form H J ⊗ S ⊗ J̄ and H J ⊗ S ⊗ J̄ ⊗ J̄. Here S and J̄ are new soft and jet functions, whose form is derived. These contributions arise whenever the photon couples to light partons instead of coupling directly to the effective weak interaction. The new contributions appear first at order 1/m_b and are related to operators other than Q_7γ in the effective weak Hamiltonian. They give rise to non-vanishing 1/m_b corrections to the total decay rate, which cannot be described using a local operator product expansion. A systematic analysis of these effects is performed at tree level in hard and hard-collinear interactions. The resulting uncertainty on the decay rate defined with a cut E_γ > 1.6GeV is estimated to be approximately ±5%. It could be reduced by an improved measurement of the isospin asymmetry δ_0- to the level of ±4%. We see no possibility to reduce this uncertainty further using reliable theoretical methods.