Systematic study of the validity of the eikonal model including uncertainties

Abstract: Nuclear reactions at intermediate beam energies are often interpreted using the eikonal model. In the analysis of complex reaction probes, where few-body reaction methods are needed, the eikonal method may be used as an efficient way for describing the fragment-target reaction process. In this work, we perform a systematic study to test the validity of the eikonal approximation for nucleon-nucleus reactions. We also quantify uncertainties due to the nucleon optical potential on reaction observables. We inspect the validity of the eikonal model and its semiclassical correction by comparing it to exact solutions (obtained from solving the optical model equation with a finite differences method) for a wide range of reactions. We also study the effect of relativistic corrections, both kinematic and dynamic, by effectively incorporating the relativistic effects at intermediate energies. The uncertainties from a Bayesian global optical potential (KDUQ) are propagated to the observables of interest. Our study includes neutron and proton reactions on $^{27}$Al, $^{40}$Ca, $^{90}$Zr and $^{208}$Pb, for a wide range of energies $E_{lab}=0-400$ MeV. Our results show that for the proton absorption cross section, the eikonal model can be used down to around $60$ MeV and the semiclassical correction extends its use to $30$ MeV. However, the validity of the eikonal model for the neutron total cross section only goes down to $\approx120$ MeV, a range extended to $\approx 50$ MeV when using the semiclassical correction. We find the semi-classical correction to the eikonal model to be less effective in describing the angular distributions. The $1\sigma$ uncertainty intervals on the observables we studied is less than $5$% for most of the energies considered, but increases rapidly for higher energies, namely energies outside the range of KDUQ ($E_{lab}>200$ MeV).