Near threshold hyperonproduction in the channels pp → pK+Λ and pp → pK+Σ0 at the experiment COSY-11

At the internal experiment COSY-11 the near threshold production of the two neutral hyperons Λ and Σ0 has been investigated in proton-proton collisions. In the measured energy region of up to 13 MeV excess energy it was found that the cross section of the Λ-production excels that of the Σ0-production by a factor of 28. This ratio of the cross sections is in contrast to measurements at higher excess energies, where a ratio of 2-3 has been determined. As one explanation of this result a NΣ→NΛ-conver sion is discussed, which is known in the literature as the so called cusp-effect. At near threshold measurements of the Σ0 -production simultaneously the lighter Λ-particle is produced. These reactions at about 80 MeV excess energies were also analysed and compared to other measurements at similar excess energy. (orig.)