Neutron densities from a global analysis of medium-energy proton-nucleus elastic scattering

A new method for extracting neutron densities from intermediate-energy elastic proton-nucleus scattering observables uses a global Dirac phenomenological approach based on the relativistic impulse approximation. Datasets for Ca-40, Ca-48, and Pb-208 in the energy range from 500 MeV to 1040 MeV are considered. The global fits are successful in reproducing the data and in predicting datasets not included in the analysis. Using this global approach, energy-independent neutron densities are obtained. The vector point proton density distribution rho(v)(p) is determined from the empirical charge density after unfolding the proton form factor. The other densities, rho(v)(n), rho(s)(p), rho(s)(n), are parametrized. This work provides energy-independent values for the rms neutron radius R-n and the neutron skin thickness S-n, in contrast to the energy-dependent values obtained by previous studies. In addition, the results presented in this paper show that the expected rms neutron radius and the skin thickness for Ca-40 are accurately reproduced. The values of R-n and S-n obtained from the global fits that we consider to be the most reliable are given as follows: for Ca-40, 3.314>R-n>3.310 fm and -0.063>S-n >-0.067 fm; for Ca-48, 3.459>R-n>3.413 fm and 0.102>S-n>0.056 fm; and for Pb-208, 5.550>R-n>5.522 fm and 0.111>S-n>0.083 fm. These values are in reasonable agreement with nonrelativistic Skyrme-Hartree-Fock models and with relativistic Hartree-Bogoliubov models with density-dependent meson-nucleon couplings. The results from the global fits for Ca-48 and Pb-208 are generally not in agreement with the usual relativistic mean-field models.