Non-factorizable contributions in B decays revisited

B̄ → Dπ, Dπ, J/ψK̄ and J/ψπ decays are studied by decomposing their amplitude into a sum of factorizable and non-factorizable ones. The former is estimated by using the naive factorization while the latter is calculated by using a hard pion approximation in the infinite momentum frame. Although experiments provide only upper bounds of the branching ratios for the color suppressed B̄0 → D0π0 and D∗0π0 decays, consistency among the branching ratios for the B̄ → Dπ and Dπ decays including the charged modes leads to phenomenologically allowed lower limits of B(B̄0 → D0π0) and B(B̄0 → D∗0π0). Our result is compared with the phenomenologically estimated branching ratios as well as the measured ones. As the consequence, it is seen that the non-factorizable amplitude is rather small in the color favored B̄ → Dπ and Dπ decays but can still efficiently interfere with the main (factorized) amplitude. In the color suppressed B̄ → J/ψK̄ and J/ψπ decays, non-factorizable contribution is more important. A sum of the factorized and non-factorizable amplitudes can improve the result from the factorization, although it is sensitive to model dependent values of form factors involved.