Lattice calculation of the penguin diagram decayB→K*γ

We calculate the leading-order matrix element for the decay B→${\mathit{K}}^{\mathrm{*}}$γ in the quenched approximation of lattice QCD on a ${24}^3$×48 lattice at β=6.2, using an O(a)-improved fermion action. Extrapolating to physical quark masses gives an on-shell form factor of ${\mathit{T}}_1$(${\mathit{q}}^2$=0)=0.${15}_{\mathrm{-}14}^{+12\mathrm{}}$ (stat). We find ${\mathit{T}}_1$ is approximately independent of the spectator quark mass and extract ${\mathit{T}}_1$(${\mathit{q}}^2$=0)=0.${15}_{\mathrm{-}4}^{+5\mathrm{}}$ if this independence is assumed. 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)×${10}^{\mathrm{-}5}$.