Helicity form factors for $D_{(s)} \to A \ell \nu$ process in the light-cone QCD sum rules approach

The helicity form factors of the $D_{(s)}\to A \ell^+ \nu$ with $A=a_{1}^{-}, a_{1}^{0}, b_{1}^{-}, b_{1}^{0}, K_{1}(1270)$ and $K_{1}(1400)$ are calculated in the light-cone sum rules approach, up to twist-3 distribution amplitudes of the axial vector meson $A$. In the helicity form factors parametrization the unitarity constraints are applied to the fitting parameters. In addition, the effects of the low-lying resonances are included in series expansions of aforementioned form factors. The properties of the $D_{(s)}\to A \ell^+ \nu$ semileptonic decays are studied by extending the form factors to the whole physical region of $q^2$. For a better analysis, a comparison is also made between our results and the predictions obtained using transition form factors via LCSR, 3PSR and CLFQM methods.