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.

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The total cross section of the reaction pp->ppK+K- has been measured at excess energies Q=10 MeV and 28 MeV with the magnetic spectrometer COSY-11. The new data show a significant enhancement of the total cross section compared to pure phase space expectations or calculations within a one boson exchange model. In addition, we present invariant mass spectra of two particle subsystems. While the K+K- system is rather constant for different invariant masses, there is an enhancement in the pK- system towards lower masses which could at least be partially connected to the influence of the Lambda(1405) resonance.Comment: accepted for publication in Phys. Lett.

The Σ + hyperon production was measured at the COSY-11 spectrometer via the pp → nK + Σ + reaction at excess energies of Q = 13 MeV and Q = 60 MeV. These measurements continue systematic hyperon production studies via the pp → pK + Λ / Σ 0 reactions where a strong decrease of the cross section ratio close-to-threshold was observed. In order to verify models developed for the description of the Λ and Σ 0 production we have performed the measurement on the Σ + hyperon and found unexpectedly that the total cross section is by more than one order of magnitude larger than predicted by all anticipated models.After the reconstruction of the kaon and neutron four momenta, the Σ + is identified via the missing mass technique. Details of the method and the measurement will be given and discussed in view of theoretical models.

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Measurements of the η meson production with a polarised proton beam in the reaction pp → ppη have been carried out at an excess energy of Q = 40 MeV. The dependence of the analysing power Ay on the polar angle θ * q of the η meson in the center of mass system (CMS) has been studied. The data indicate the possibility of an influence of p-and d-waves to the close to threshold η production.