Radiative proton capture on 15N within effective field theory

The astrophysical S factor for the radiative proton capture process on the 15N nucleus, i.e., 15N(p, gamma ) 16O, at stellar energies are studied within the framework of the cluster effective field theory. The thermonuclear 15N(p, gamma ) 16O reaction links the type-I to type-II cycles of the carbon-nitrogen-oxygen cycle and affects the abundances of elements in the universe. For investigating this reaction in the effective field theory formalism, we first construct an effective Lagrangian that is appropriate for this reaction at low energies. Since the intermediate excited states of the 16O nucleus have a crucial role in this reaction, we include these resonances in the formalism. The corresponding radiative capture amplitudes and cross section are calculated, which lead to the astrophysical S factor. The low-energy constants introduced in the effective Lagrangian are determined by fitting the theoretical results to the observed S factors in the range of 130 keV < Ep < 2500 keV using three different experimental data sets. Considering the recent data sets, we obtain S(0) = 29.8-34.1 keV b, which is in a good agreement with the estimates from R-matrix approaches in the literature. The values of S at the Gamow energy are found to be larger than S(0) values by about 10%.