Long-Distance dominance of the CP asymmetry in B̄→X _s,dγ decays

We show that in the standard model the parametrically leading (by a factor 1/α_s) contribution to the inclusive CP asymmetry in B̄→X_s,dγ decays arises from a long-distance effect in the interference of the electromagnetic dipole amplitude with the amplitude for an up-quark penguin transition accompanied by soft gluon emission. Using model estimates for the associated hadronic parameter Λ17u, we predict a value in the range -0.6%<AX_sγSM<2.8%. In view of current experimental data, a future precision measurement of the flavor-averaged CP asymmetry would signal the presence of new physics only if a value below -2% was found. A cleaner probe of new physics is offered by the difference of the CP asymmetries in charged versus neutral B̄→X_sγ decays.