Constraining the 6.05 MeV 0^+ and 6.13 MeV 3^- cascade transitions in the ¹²C(α,γ)¹⁶O reaction using the Asymptotic Normalization Coefficients

Background. The $^{12}$C$(\alpha,\gamma)^{16}$O reaction plays a fundamental role in astrophysics and needs to be known with accuracy better than 10%. Cascade $\gamma$ transitions through the excited states of $^{16}$O are contributing to the uncertainty. Purpose. To constrain the contribution of the 0$^+$ (6.05 MeV) and 3$^-$ (6.13 MeV) cascade transitions. Method. The $\alpha$ Asymptotic Normalization Coefficient for these states were measured using the $\alpha$-transfer reaction $^{12}$C$(^6$Li,$d)^{16}$O at sub-Coulomb energies. Results. The 0$^+$ and 3$^-$ cascade transitions contribution was found to be 1.96$\pm$0.3 keV b and 0.12$\pm$0.04 keV b for destructive interference of the direct and resonance capture and 4.36$\pm$0.45 keV b and 1.44$\pm$0.12 keV b for constructive interference respectively. Conclusions. The combined contribution of the 0$^+$ and 3$^-$ cascade transitions to the reaction at 300 keV does not exceed 4%. Significant uncertainties have been dramatically reduced.