Associated strangeness production at threshold

The associated strangeness dissociation at threshold has been studied at the COSY-11 facility measuring the hyperon and the K+K- meson pair production. Measurements of the near-threshold Λ and Σ0 production via the pp→pK+Λ/Σ0 reaction (S.Sewerin et al., Phys. Rev. 83, 682 (1999)) at COSY-11 have shown that the Λ/Σ0 cross-section ratio exceeds the value at high excess energies (Q≥300 MeV (A. Baldini et al., Total Cross-Sections for Reactions of High-Energy Particles, Landolt-Boernstein, New Series, Vol. I/12 (Springer, Berlin, 1988))) by an order of magnitude. For a better understanding additional data have been taken between 13 MeV and 60 MeV excess energy. The near-threshold production of the charged kaon-antikaon pair is related to the discussion about the nature of the scalar states in the 1 GeV/c2 mass range, i.e. the f0(980) and a0(980) (O. Krehl, R. Rapp, J. Speth, Phys. Lett. B 390, 23 (1997)). The interpretation as a K K molecule is strongly dependent on the K-K interaction which can be studied via the production channel. A first total cross-section value on the reaction pp→pp K + K- at an excess energy of 17 MeV (C. Quentmeier et al., Phys. Lett. B 515, 276 (2001)), i.e. below the φ production threshold, was measured. (orig.)