Quark mass effects in B -> Xs gamma

The charm-loop contribution to B -> Xs gamma is found to be numerically dominant and very stable under logarithmic QCD corrections. The strong enhancement of the branching ratio by QCD logarithms is mainly due to the b-quark mass evolution in the top-quark sector. These observations allow us to achieve better control over residual scale dependence at the next-to-leading order. Furthermore, we observe that the sensitivity of the matrix element < Xs gamma | (sc)_(V-A)(cb)_(V-A) | b > to mc/mb is the source of a sizeable uncertainty that has not been properly taken into account in previous analyses. Replacing mc^pole/mb^pole in this matrix element by the more appropriate mc(mu)/mb^pole with mc < mu < mb causes an 11% enhancement of the SM prediction for BR[B -> Xs gamma]. For E_gamma > 1.6 GeV in the B-meson rest frame, we find BR[ B -> Xs gamma]_{E_gamma > 1.6 GeV} = (3.60 +_ 0.30) * 10^-4. The difference between our result and the current experimental world average is consistent with zero at the level of 1 sigma. We also discuss the implementation of new physics effects in our calculation. The lower bound on the charged Higgs boson mass in 2HDM(II) is found the be higher than 350 GeV.