Radiative proton capture on ${}^{12}\mathrm{C}$ in cluster effective field theory

Abstract

Radiative proton capture on ${}^{12}\mathrm{C}$ to the ground state of ${}^{13}\mathrm{N}$ is calculated in cluster effective field theory through next-to-leading order. At low energy the reaction is dominated by $s_{1/2}\to p_{1/2}$ $E1$ capture, which we treat in the long-wavelength limit. Its amplitude is strongly hindered by destructive interference between the contribution fixed by gauge invariance and a counter-term $E1$ current, making the capture particularly sensitive to subleading terms. We fit the recent data below $0.95$~MeV, determining the $E1$ transition counter-term coefficients together with the ground-state asymptotic normalization coefficient and the $\frac{1}{2}^+$ resonance and shape parameters. We obtain $S(0)=1.34\pm0.07$~keV\,b and $S(25~\mathrm{keV})=1.43\pm0.07$~keV\,b, the latter in agreement with recent $R$-matrix extrapolations. We also estimate the effects of the omitted $p$-wave $M1$ capture and $\frac{3}{2}^-$ resonance tail, and find that the fitted asymptotic normalization coefficient moves substantially while the $S$ factors remain almost unchanged.