Folding model study ofα−pscattering: Systematics of elastic scattering, effective interaction, and inelastic excitation ofN*resonances

Abstract

For a description of elastic and inelastic $\ensuremath{\alpha}\ensuremath{-}p$ scattering exciting ${N}^{*}$ resonances optical potentials and transition potentials were derived by folding nucleon and nucleus mass densities with a variable range effective interaction. For elastic $\ensuremath{\alpha}\ensuremath{-}p$ scattering in forward direction, a reasonable description of essentially all data has been obtained from low energies up to the GeV region. Also, $\ensuremath{\alpha}$ scattering from ${}^{4}\mathrm{He}$ and ${}^{12}\mathrm{C}$ is quite well described with potentials, which indicate that the used folding method is a valid approach for the systems in question. The strong energy dependence of the deduced potentials can be accounted for by a sum of scalar and vector meson-exchange potentials and a soft Pomeron-exchange contribution. The scalar meson-exchange potential falls off rapidly with energy and has a large radius in agreement with theoretical predictions. Consistent with the flavor SU(3) quark model, the vector-meson coupling is rather weak in the central potential, but is strong in the spin-orbit potential, for which a soft Pomeron contribution is negligible. The differences between the deduced $\ensuremath{\alpha}\ensuremath{-}p$ and nucleon-nucleon $(\mathrm{NN})$ potentials are understood; further, an excellent description of the energy dependence of the s-wave $\mathrm{NN}$ amplitudes is obtained in the folding model framework. Distorted wave Born approximation calculations for inelastic $\ensuremath{\alpha}\ensuremath{-}p$ scattering show a t-dependence of the $L=0$ cross section consistent with empirical form factors. Absolute yields for excitation of the resonances ${P}_{11}(1440),$ ${D}_{13}(1520)$ and ${F}_{15}(1680)$ were calculated, using resonance shapes from $\ensuremath{\pi}\ensuremath{-}N$ scattering. A quantitative description of the data at ${E}_{\ensuremath{\alpha}}=4.2\mathrm{GeV}$ is obtained using fluid-dynamical transition densities and strengths exhausting large fractions of scalar energy weighted sum rules. The rather pure scalar (non-spin-isospin-flip) character of these excitations and the observed cross sections are in severe conflict with the constituent quark model. Finally, a prediction is made for $p\ensuremath{-}\ensuremath{\alpha}$ scattering at an incident energy of ${E}_{p}=2.2\mathrm{GeV},$ which yields strongly increased cross sections for ${N}^{*}$ excitations.