Nonfactorizable contributions to the decay modeD0→K0K0

We point out that the decay mode $D^0\to K^0{K}^0$ has no factorizable contribution. In chiral perturbation language, treating $D^0$ as heavy, the $\mathcal{O}\mathrm{}\left(p\right)\mathrm{}$ contribution is zero. We calculate the nonfactorizable chiral loop contributions of order $\mathcal{O}{(p}^3).\mathrm{}$ Then, we use a heavy-light type chiral quark model to calculate nonfactorizable tree level terms, also of order $\mathcal{O}{(p}^3),$ proportional to the gluon condensate. A priori, chiral loops are not expected to give good precision because the energy release in this decay is almost 800 MeV. Still, we find that both the chiral loops and the gluon condensate contributions are of the same order of magnitude as the experimental amplitude.