Comparison of π+ and π− elastic scattering on the isotopic pair 16O 18O in the resonance region

Abstract

The experimental comparative study of π + and π − elastic scattering from the isotopes 16O and 18O at 163 MeV performed at SIN is analyzed. Complementary information is provided by the measurements of elastic angular distributions on 16O and 18O at several incident pion energies performed at SIN and by the total cross-section difference measurements at LAMPF. Using a momentum-space first-order nonstatic optical potential, we present a study of these data with several phenomenological or theoretical proton and neutron distributions for each isotope. In the framework of the model, the position of the first minima, the relative differences and isotopic shifts for the differential cross sections, as well as the isotopic shifts for the total cross sections are well reproduced by using harmonic oscillator densities with identical neutron and proton distributions for 16O, the same distributions for the proton density in 18O, while the neutron distribution in 18O is more extended by about 0.15 fm, in agreement with theoretical estimates of the difference between the neutron and proton rms radii in 18O. However, almost similar agreement with the data can be obtained with phenomenological densities, where the proton and neutron distributions are identical in each isotope. More precise experiments at several incident pion energies, and probably improved theoretical analyses, are needed before unambiguous conclusions may be extracted.