Rare decay [ital B][r arrow][ital K][sup ][gamma]: A more precise calculation

Efforts to predict the rare exclusive decay [ital B][r arrow][ital K][sup *][gamma] from the well-known inclusive decay [ital b][r arrow][ital s][gamma] are frustrated by the effect of the large recoil momentum. We show how to reduce the large uncertainty in calculating this decay by relating [ital B][r arrow][ital K][sup *][gamma] to the semileptonic process [ital B][r arrow][rho][ital e][bar [nu]] using the heavy-quark symmetry in [ital B] decays and SU(3) flavor symmetry. A direct measurement of the [ital q][sup 2] spectrum for the semileptonic decay can provide accurate information for the exclusive rare decay.