Radiative decay of the sub-threshold $1_1^-$ and $2_1^+$ states of $^{16}$O in cluster effective field theory

Abstract

Radiative decay of the sub-threshold $1_1^-$ and $2_1^+$ states of $^{16}$O is studied in cluster effective field theory. The wave function normalization factors for initial and final states of the radiative decay amplitudes are deduced by using the phase shift data of elastic $\alpha$-$^{12}$C scattering for $l=0,1,2$, which are related to the asymptotic normalization coefficients of $0_1^+$, $1_1^-$, $2_1^+$ states of $^{16}$O for the two-body $\alpha$-$^{12}$C channel; then only one unfixed parameter remains in each of the radiative decay amplitudes. We fit the parameters to the experimental radiative decay rates and apply the fitted parameters to the study of radiative $\alpha$ capture on $^{12}$C, $^{12}$C($\alpha$,$\gamma$)$^{16}$O. The order of magnitude of astrophysical $S$ factors of $E1$ and $E2$ transitions of $^{12}$C($\alpha$,$\gamma$)$^{16}$O is reproduced compared to the experimental data, and we discuss an improvement in the calculation of $S$ factors of $^{12}$C($\alpha$,$\gamma$)$^{16}$O.