LATTICE CALCULATION OF THE PENGUIN DIAGRAM DECAY B-]K-ASTERISK-GAMMA

We calculate the leading-order matrix element for the decay B --> K ga mma in the quenched approximation of lattice QCD on a 24(3) x 48 latti ce at beta = 6.2, using an O(a)-improved fermion action. Extrapolating to physical quark masses gives an on-shell form factor of T1(q2=0) = 0.15(-14)+12 (stat). We find T1 is approximately independent of the sp ectator quark mass and extract T1(q2=0) = 0.15(-4)+5 if this independe nce is assumed. We find the results to be consistent (in the standard model) with the CLEO experimental branching ratio of B(B --> K-gamma) = (4.5 +/- 1.5 +/-0.9) x 10(-5).