THE ROLE OF THE DELTA(1920) RESONANCE FOR KAON PRODUCTION IN HEAVY-ION COLLISIONS

The long mean free path of K+ mesons in nuclear matter make this particle a suitable messenger for the dynamics of nucleus-nucleus reactions at intermediate energies (100 MeV to 3 GeV per nucleon). A prerequisite for this is the knowledge of the elementary production cross sections piN --> SIGMAK. Here these cross sections are studied for the first time with the explicit inclusion of the relevant baryon resonances up to 2 GeV as intermediate states. The baryon resonances - N(1710) I(J(p)) = 1/2(1/2+), N(1720) 1/2(3/2+) and DELTA(1920) 3/2(3/2+) - are taken into account coherently in the calculations of the piN --> SIGMAK process. Also K*(892)1/2(1-) vector meson exchange is included. It is shown that the total cross sections for different channels of the piN --> SIGMAK reactions, i.e. pi+ p --> SIGMA+K+, pi-P --> SIGMA- K+, pi+n --> SIGMA0K+ (pi- p --> SIGMA-K+) and pi0 p --> SIGMA0K+ differ not only by absolute values but also by their energy dependence. This shape differences are due to the mixture of the isospin I = 3/2 DELTA(1920) with isospin I = 1/2 nucleon resonances. However, this I = 3/2 resonance does not give a contribution to the piN --> LAMBDAK reactions. So the shapes of the total cross sections piN --> LAMBDAK for different isospin projections are the same. In spite of this, such cross sections averaged over different isospin projections in the same multiplet are often used for studies of kaon production in heavy ion collisions. Here we give an explicit separate parameterization of the cross sections for each channel piN --> SIGMAK, which are calculated in this work.