Investigations of channel radius sensitivity in R-matrix analysis

The phenomenological R-matrix is often the tool of choice for evaluating cross sections and making predictions for unmeasured observables, e.g. for the very low-energy cross sections needed in astrophysical calculations. According to formal R-matrix theory, the channel radius should be large enough so that at and beyond the channel radius, nuclear forces are negligible and Coulomb wave functions are a good approximation. In practice, phenomenological R-matrix fits must use channel radii that enclose most but not all of the nuclear interactions, in order to minimize the contributions of background poles. How much this approximation actually matters is an open question. We study the sensitivity of conclusions regarding the choice channel radius by fitting α+¹²C elastic scattering, where high-quality experimental data are available. The sensitivity to the channel radius in each important partial wave is investigated independently. We will also present results of fits that include the tail of a phenomenological nuclear potential extending beyond the channel radius -- the so-called hybrid model. This work will also provide insight into how accurately the Asymptotic Normalization Constants (ANCs) of the near-threshold bound states of ¹⁶O can be extracted from scattering data.