Semileptonic $D_{q}\to K_{1}\ell \nu$ and nonleptonic $D\to K_1 \pi$   decays in three--point QCD sum rules and factorization approach
  We analyze the semileptonic $D_{q}\to K_1 \ell\nu$ transition with $q=u, d, s$, in the framework of the three--point QCD sum rules and the nonleptonic $D\to K_1 \pi$ decay within the QCD factorization approach. We study $D_{q}$ to $K_1(1270)$ and $K_1(1400)$ transition form factors by separating the mixture of the $K_1(1270)$ and $K_1(1400)$ states. Using the transition form factors of the $D\to K_1 $, we analyze the nonleptonic $D\to K_1 \pi$ decay. We also present the decay amplitude and decay width of these decays in terms of the transition form factors. The branching ratios of these channel modes are also calculated at different values of the mixing angle $\theta_{K_1}$ and compared with the existing experimental data for the nonleptonic case. 
R. Khosravi, K. Azizi, N. Ghahramany