Renormalization-Group Improved Calculation of the B->Xs+gamma Branching Ratio

Using results on soft-collinear factorization for inclusive B-meson decay distributions, a systematic study of the partial $B\to X_s\gamma$ decay rate with a cut $E_\gamma>E_0$ on photon energy is performed. For values of $E_0$ below about 1.9 GeV, the rate can be calculated without reference to shape functions using a multi-scale operator product expansion (MSOPE). The transition from the shape-function region to the MSOPE region is studied analytically. The resulting prediction for the $B\to X_s\gamma$ branching ratio depends on three large scales: $m_b$, $\sqrt{m_b\Delta}$, and $\Delta=m_b-2E_0$. Large logarithms associated with these scales are resummed at next-to-next-to-leading logarithmic order. While power corrections in $\Lambda_{QCD}/\Delta$ turn out to be small, the sensitivity to the scale $\Delta\approx 1.1$ GeV (for $E_0\approx 1.8$ GeV) introduces significant perturbative uncertainties, which so far have been ignored. The new theoretical prediction for the $B\to X_s\gamma$ branching ratio with $E_\gamma>1.8$ GeV is $Br(B\to X_s\gamma)=(3.44\pm 0.53\pm 0.35)\times 10^{-4}$, where the first error is an estimate of perturbative uncertainties and the second one reflects uncertainties in input parameters. With this cut $(92_{-10}^{+7}\pm 1)%$ of all events are contained. The implications of larger theory uncertainties for New Physics searches are briefly explored with the example of the type-II two-Higgs-doublet model, for which we find that the lower bound on the charged-Higgs mass is strongly reduced compared to previous estimates, to slightly below 200 GeV at 95% confidence level.