Delta excitation in proton-proton and proton-nucleus collisions

Abstract

The delta production in proton-proton and proton-nucleus collisions is discussed. First a t-matrix for the elementary process pp → nΔ ++ is constructed using the one-pion exchange potential for the transition interaction and including the effect of elastic and other channels through optical potentials. The predictions of this t-matrix are compared with the experimental cross sections over a wide energy range. In the Born approximation the t-matrix is also constructed in the quark model. For use in the delta production reactions in nuclei, the t-matrix based on the one-pion exchange potential is parametrized in a convenient form. This parametrized t-matrix (PTM) is complex. Its real part resembles the one-pion exchange potential with a reduced length parameter in the central as well as tensor parts and a Landau-Migdal term in the central part. The PTM is subsequently used to analyse the experimental data on p( 3He, t)Δ ++ and 6Li(p, Δ ++) 6He reactions in the single scattering approximation. The experimental data are reproduced well. In a collision process the delta may be excited in the target nucleus or it may be excited in the projectile. A study on the relative contribution of these two processes is presented. The excitation of a nucleon to delta can also be initiated by the exchange of a rho meson. We study the role of rho exchange in charge exchange reactions, p(n, p)n and p(p, n)Δ ++. Comparison of the calculated cross sections with the corresponding experimental data indicates that, unlike the ϱNN coupling, the strength of the ϱNΔ coupling could be much weaker. The effect of distortion is studied next from threshold to around 3 GeV beam energy in the (p, Δ ++) reaction and at 1.5 GeV/ c beam momentum in the (p, nΔ ++) reaction. We use the partial wave method as well as the eikonal approximation for a comparative study. It is found that the distortion mainly reduces the magnitudes of the cross sections in the (p, Δ ++) reaction without introducing any change in the shapes. In the (p, nΔ ++) reaction, however, it changes the magnitude as well as the shape of the neutron energy spectrum. Finally, using the relativistic Lagrangian for the πNN and πNΔ couplings, uncertainties due to use of nonrelativistic static version of the NN → NΔ potential in the study of the Δ-production in nuclei are studied.