Relativistic effects on polarization transfer observables for quasielastic proton-nucleus scattering

Abstract

Within the relativistic plane-wave impulse approximation, the sensitivity of complete sets of quasielastic proton-nucleus spin observables is investigated with respect to relativistic effects, the importance of exchange contributions to the NN amplitudes, the form of the \ensuremath{\pi}N vertex, and spin-orbit distortions. Calculations are performed at laboratory energies ranging from 135 to 500 MeV and for target nuclei ranging from ${\mathrm{C}}^{12}$ to ${\mathrm{Pb}}^{208}$ at a fairly large fixed momentum transfer of 1.97 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$. Of all the spin observables, only ${\mathrm{A}}_{\mathrm{y}}$ exhibits a clear relativistic effect at the highest energy, as has been previously found. However, at the lower energies (200 MeV and below) the relativistic effect shifts to two other polarization transfer observables, mainly ${\mathrm{D}}_{\mathrm{s}\ensuremath{'}\mathrm{s}}$ and to a lesser extent ${\mathrm{D}}_{\mathrm{l}\ensuremath{'}\mathrm{s}}$. In addition, it is found that ${\mathrm{D}}_{\mathrm{n}\ensuremath{'}\mathrm{n}}$ is extremely sensitive to the pseudoscalar versus pseudovector ambiguity of the \ensuremath{\pi}N vertex, whereas ${\mathrm{D}}_{\mathrm{s}\ensuremath{'}\mathrm{l}}$ and ${\mathrm{D}}_{\mathrm{l}\ensuremath{'}\mathrm{l}}$ are sensitive to exchange contributions to the NN amplitudes. Compared to the latter, the effects of spin-orbit distortion are not insignificantly small and had to be corrected for. This investigation stresses the urgent need for measurements of the quasielastic spin observables at these low energies.