Dynamics of strangeness production in the near threshold nucleon-nucleon collisions

We investigate the associated strangeness Lambda K+ and Sigma0 K+ productions in proton-proton collisions within an effective Lagrangian model. The initial interaction between the two nucleons is modeled by the exchange of pi, rho, omega, and sigma mesons and the strangeness production proceeds via excitations of N*(1650), N*(1710), and N*(1720) baryonic resonances. The parameters of the model at the nucleon-nucleon-meson vertices are determined by fitting the elastic nucleon-nucleon scattering with an effective interaction based on the exchange of these four mesons, while those at the resonance vertices are calculated from the known decay widths of the resonances and from the vector meson dominance model. Experimental data taken recently by the COSY-11 collaboration are described well by this approach. The one-pion-exchange diagram dominates the production process at both higher and lower beam energies. The excitation of the N*(1650) resonance dominates both the production channels at near threshold energies. Our model with final state interaction effects among the outgoing particles included within the Watson-Migdal approximation, is able to explain the observed beam energy dependence of the ratio of the total cross sections of these two reactions.