We calculate the leading-order matrix element for exclusive decays of b→sγ in the quenched approximation of lattice QCD on a ${24}^3$×48 lattice at β=6.2, using an O(a)-improved fermion action. The matrix element is used to extract the on-shell form factor ${\mathit{T}}_1$(${\mathit{q}}^2$=0) for B→${\mathit{K}}^{\mathrm{*}}$γ and ${\mathit{B}}_{\mathit{s}}$→φγ, using two different assumptions about the ${\mathit{q}}^2$ dependence of the form factors for these decays. For B→${\mathit{K}}^{\mathrm{*}}$γ, ${\mathit{T}}_1$(${\mathit{q}}^2$=0) is determined to be 0.${159}_{\mathrm{-}33}^{+34\mathrm{}}$±0.067 or 0.${124}_{\mathrm{-}18}^{+20\mathrm{}}$±0.022 in the two cases. We find the results to be consistent (in the standard model) with the CLEO experimental branching ratio of B(B→${\mathit{K}}^{\mathrm{*}}$γ)=(4.5±1.5±0.9)×${10}^{\mathrm{-}5}$.

• Received 19 July 1994

DOI:https://doi.org/10.1103/PhysRevD.51.4955