Enhanced Electromagnetic Corrections to the Rare Decay Bs,d → μ+μ-

We investigate electromagnetic corrections to the rare B-meson leptonic decay B-s,B-d -> mu(+)mu(-) from scales below the bottom-quark mass m(b). Contrary to QCD effects, which are entirely contained in the B-meson decay constant, we find that virtual photon exchange can probe the B-meson structure, resulting in a nonlocal annihilation effect. We find that this effect gives rise to a dynamical enhancement by a power of m(b)/Lambda(QCD)and by large logarithms. The impact of this novel effect on the branching ratio of B-s,B-d -> mu(+)mu(-) is about 1 %, of the order of the previously estimated nonparametric theoretical uncertainty, and four times the size of previous estimates of next-to-leading order QED effects due to residual scale dependence. We update the standard model (SM) prediction to (B)over bar(B-s -> mu(+)mu(-))(SM) = (3.57 +/- 0.17) x 10(-9).