Quark mass effects in B̄→Xsγ Original Research Article

The charm-loop contribution to B̄→Xsγ 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 〈Xsγ|(s̄c)V−A(c̄b)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 mcpole/mbpole in this matrix element by the more appropriate mcMS(μ)/mbpole with μ∈[mc,mb] causes an 11% enhancement of the SM prediction for BR[B̄→Xsγ]. For Eγ>1.6 GeV in the B̄-meson rest frame, we find BR[B̄→Xsγ]Eγ>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 1.6σ. We also discuss the implementation of new physics effects in our calculation. The Two-Higgs-Doublet-Model II with a charged Higgs boson lighter than 450 GeV is found to be strongly disfavoured.