Threshold hyperon production in proton-proton collisions at COSY-11

The Σ+ hyperon production was measured at the COSY-11 spectrometer via the pp→nK+Σ+ reaction at excess energies of Q=13MeV and Q=60MeV. These measurements continue systematic hyperon production studies via the pp→pK+Λ/Σ0 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 Σ0 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.