Three-parton contributions to B→M1M2 annihilation at leading order

We compute annihilation amplitudes for charmless B decays that are proportional to the three-parton twist-3 light meson distribution amplitude ϕ3M(x1,x2) with an active gluon. Due to an enhancement from a quark propagator at the scale p2∼mbΛQCD these terms occur at the same parametric order in αs(mb) and 1/mb as the known leading order annihilation involving fB and twist-2 meson distributions. With our calculation the leading order annihilation amplitude is now complete. At lowest order in αs the new amplitudes are real and only O5-8 contribute. Using simple models we find that the three-parton and two-parton terms are of comparable size.