Rare B meson decays on the lattice

We discuss a framework for the measurement of the B → K * transition form factors in lattice simulations, when the K * eventually decays. The possible mixing of πK and ηK states is considered. We reproduce the two-channel analogue of the Lellouch-Luscher formula, which allows one to extract the B → K * l + l − decay amplitude in the low-recoil region. Since the K * is a resonance, we provide a procedure to determine the form factors at the complex pole position in a process-independent manner. The infinitely-narrow width approximation of the results is also studied.