Weak interference between the 1$^-$ states in the vicinity of $\alpha$-particle threshold of $^{16}$O

Abstract

The subthreshold 1$^-_1$ state at an excitation energy $E_x = 7.12$ MeV in $^{16}$O has been believed to enhance the $S$-factor of $^{12}$C($\alpha$,$\gamma$)$^{16}$O. The enhancement seems to originate from strong interference between 1$^-_1$ and 1$^-_2$ ($E_x\approx 9.6$ MeV) in the vicinity of the $\alpha$-particle threshold. However, weak interference between them and a resulting small $E$1 $S$-factor are exemplified with $R$-matrix theory. Including a higher-order correction of the resonance parameters, the present example appears to reproduce the experimental data consistently. It would therefore be possible that the $E$1 $S$-factor is reduced at low energies.