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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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The proton-proton and proton-η ′ invariant mass distributions have been determined for the pp → ppη ′ reaction at an excess energy of Q = 16.4 MeV. The measurement was carried out using the COSY-11 detector setup and the proton beam of the cooler synchrotron COSY. The shapes of the determined invariant mass distributions are similar to those of the pp → ppη reaction and reveal an enhancement for large relative proton-proton momenta. This result, together with the fact that the proton-η interaction is much stronger that the proton-η ′ interaction, excludes the hypothesis that the observed enhancement is caused by the interaction between the proton and the meson.

Abstract. Measurements of the pp → pK+ Λ reaction at Tp = 2.28 GeV have been carried out at COSY-TOF. In addition to the Λp FSI and N * resonance excitation effects a pronounced narrow structure is observed in the Dalitz plot and in its projection on the pΛ-invariant mass. The strongly asymmetric structure appears at the pp → N K + Σ threshold and is interpreted as ΣN cusp effect. The observed width of about 20 MeV/c 2 is substantially broader than anticipated from previous measurements as well as theoretical predictions. Angular distributions of this cusp structure are shown to be dissimilar to those in the residual pK + Λ channel, but similar to those observed in the pK + Σ 0 channel.

Abstract. The possibility to determine the pΛ scattering length from the final-state interaction in the reaction pp → pK + Λ is investigated experimentally. From a transversely polarized measurement, the K + analyzing power (AN ) which, in principle, allows one to extract the spin triplet scattering length is studied. An unexpected energy dependence of the forward/backward symmetric part of AN is found. The influence of N * resonances on the pΛ invariant mass spectrum is investigated by exploiting the large acceptance for the process pp → pK− and is found to be the main source of uncertainty for determining the pΛ scattering length.PACS. 13.75.-n Hadron-induced low-and intermediate-energy reactions and scattering (energy ≤ 10 GeV) -13.75.Ev Hyperon-nucleon interactions -25.40.Ve Other reactions above meson production thresholds (energies > 400 MeV)

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