On the colour suppressed decay modes B0→Ds+Ds− and Bs0→D+D−

We point out that the decay modes B0→Ds+Ds− and Bs0→D+D− have no factorized contributions. At quark level these decays can only proceed through the annihilation mechanism, which in the factorized limit gives zero amplitude due to current conservation. In this Letter, we identify the dominating non-factorizable (colour suppressed) contributions in terms of two chiral loop contributions and one soft gluon emission contribution. The latter contribution can be calculated in terms of the (lowest dimension) gluon condensate within a recently developed heavy-light chiral quark model. We find branching ratios BR(B0→Ds+Ds−) ≃7×10−5 and BR(B0s→D+D−) ≃1×10−3.