For a description of elastic and inelastic $\alpha -p$ 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 $\alpha -p$ scattering in forward direction, a reasonable description of essentially all data has been obtained from low energies up to the GeV region. Also, $\alpha$ 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 $\alpha -p$ and nucleon-nucleon $\mathrm{}\left(\mathrm{NN}\right)\mathrm{}$ 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 $\alpha -p$ scattering show a t-dependence of the $L=0\mathrm{}$ cross section consistent with empirical form factors. Absolute yields for excitation of the resonances $P_{11}\mathrm{}\left(1440\right),$ $D_{13}\mathrm{}\left(1520\right)\mathrm{}$ and $F_{15}\mathrm{}\left(1680\right)\mathrm{}$ were calculated, using resonance shapes from $\pi -N$ scattering. A quantitative description of the data at $E_{\alpha }=4.2\mathrm{}\mathrm{GeV}$ 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 $p-\alpha$ scattering at an incident energy of $E_p=2.2\mathrm{}\mathrm{GeV},$ which yields strongly increased cross sections for $N^{*}$ excitations.

• Received 10 July 2002

DOI:https://doi.org/10.1103/PhysRevC.67.064001