The semileptonic decays B_c^* to η_c ell bar{ν}_{ell} with QCD sum rules

In this article, we calculate the $B_c^* \to \eta_c$ form-factors with the three-point QCD sum rules, then study the semileptonic decays $B_c^* \to \eta_c \ell \bar{\nu}_{\ell}$. The tiny decay widths may be observed experimentally in the future at the LHCb, while the $B_c^* \to \eta_c$ form-factors can be taken as basic input parameters in other phenomenological analysis.