We calculate the leading-order matrix element for exclusive decays of $b \to s\gamma$ in the quenched approximation of lattice QCD on a $24^3\times48$ lattice at $\beta=6.2$, using an O(a)-improved fermion action. The matrix element is used to extract the on-shell form factor $T_1(q^2=0)$ for $B \to K^*\gamma$ and $B_s \to \phi\gamma$, using two different assumptions about the $q^2$ dependence of the form factors for these decays. For $B \to K^*\gamma$, $T_1(q^2=0)$ is determined to be $0.159^{ +34}_{ -33}\pm 0.067$ or $0.124^{ +20}_{ -18} \pm 0.022$ in the two cases. We find the results to be consistent (in the Standard Model) with the CLEO experimental branching ratio of $BR(B\to K^*\gamma) = (4.5 \pm 1.5 \pm 0.9) \times 10^{ -5}$.
Categories: hep-lat hep-ph