Abstract. The production of the Λ and Σ 0 hyperons has been measured via the pp → pK + Λ/Σ 0 reaction at the internal COSY-11 facility in the excess energy range between 14 and 60 MeV. The transition of the Λ/Σ 0 cross section ratio from about 28 at Q ≤ 13 M eV to the high energy level of about 2.5 is covered by the data showing a strong decrease of the ratio between 10 and 20 MeV excess energy. Effects from the final state interactions in the p − Σ 0 channel seems to be much smaller compared to the p − Λ one. Estimates of the effective range parameters are given for the N Λ and the N Σ systems.

Abstract:The energy dependence of the total cross section for the pp −→ pK + Λ reaction was measured in the threshold region covering the excess energy range up to 7 MeV.Existing model calculations describe the slope of the measured cross sections well, but are too low by a factor of two to three in rate.The data were used for a precise determination of the beam momentum of the COSY-synchrotron.

The production of η mesons has been measured in the proton-proton interaction close to the reaction threshold using the COSY-11 internal facility at the cooler synchrotron COSY. Total cross sections were determined for eight different excess energies (ǫ) in the range from ǫ = 0.5 MeV to ǫ = 5.4 MeV.The energy dependence of the total cross section is well described by the available phase-space volume weighted by FSI factors for the proton-proton and proton-η pairs.

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Threshold measurements of the associated strangeness production reactions pp → pK + Λ and pp → pK + Σ 0 are presented. Although slight differences in the shapes of the excitation functions are observed, the most remarkable feature of the data is that at the same excess energy the total cross section for the Σ 0 production appears to be about a factor of 28 smaller than the one for the Λ particle. It is concluded that strong Σ 0 p final state interactions, and in particular the ΣN → Λp conversion reaction, are the likely cause of the depletion for the yield in the Σ signal. This hypothesis is in line with other experimental evidence in the literature.