Possibility of vector meson exchange in pion production in proton-proton collisions

Abstract

In the bag model, confinement of quarks is accomplished by introduction of a boundary condition at some definite radius R, where the energy of the total system is a minimum. This minimum is, however, realtively shallow and energies for substantially different bag radii are not much larger than this minimum value. This indicates that the zero-point motion of the bag surface may be important.In this paper, quantization of the bag surface motion is carried out in a somewhat ad hoc fashion, modelled after the generator coordinate theory in nuclear physics. This procedure unifies a number of ideas previously in the literature; it stresses the anharmonicity of the collective motion. As in earlier treatments, the Roper resonance emerges as a breathing-mode type of excitation of the nucleon.The one- and two-pion decays of the Roper resonance are calculated and the widths are found to fall short of the empirical ones. It is pointed out, however, that decays involving intermediate states containing virtual ρ-mesons will enhance the widths. Pion-nucleon scattering in the P11 channel is constructed in our model and found to agree roughly with experiment. A crucial term in the driving force involves the pion coupling to the nucleon through a virtual ρ-meson.With introduction of zero-point motion of the bag surface, the notion of “bag radius” becomes dependent on precisely which moment of the radius is measured. Our development gives a model for cutting off smoothly the pion-exchange term in the nucleon-nucleon interaction.