Spin-rotation parameter Q for elastic scattering of 800 million electron-volt protons from oxygen-16, calcium-40, and lead-208

The spin-rotation parameter Q was measured for oxygen-16, calcium-40 and lead-208 using the 800 million electron-volt proton beam produced at the Clinton P. Anderson Meson Physics Facility. The experiment was carried out using the High Resolution Spectrometer equipped with a focal-plane polarimeter to determine the scattered polarization components in all three directions. These data (when combined with previous cross section and analyzing power data) determine the amplitude describing the elastic scattering of protons from these spin-zero nuclei to within an overall phase. Q is shown to be more sensitive than either the cross section or analyzing power to differences between the nonrelativistic and relativistic scattering theories that describe the proton-nucleus reaction in terms of fundamental proton-nucleon input. The nonrelativistic predictions for Q generally lie below the data but consistently have the correct structure. The relativistic predictions for the calcium-40 and lead-208 Q data are quite good (better than the nonrelativistic predictions). The relativistic predictions for the oxygen-16 Q data show much more structure than is evident in the data