Lattice calculation of the branching ratio for some of the exclusive modes of B-]S-gamma

Abstract

We calculate the leading-order matrix element for exclusive decays of b→sγ in the quenched approximation of lattice QCD on a 243×48 lattice at β=6.2, using an O(a)-improved fermion action. The matrix element is used to extract the on-shell form factor T1(q2=0) for B→K*γ and Bs→φγ, using two different assumptions about the q2 dependence of the form factors for these decays. For B→K*γ, T1(q2=0) is determined to be 0.159-33+34±0.067 or 0.124-18+20±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→K*γ)=(4.5±1.5±0.9)×10-5.