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The low emittance and small momentum spread of the proton and deuteron beams of the Cooler Synchrotron COSY combined with the high mass resolution of the COSY-11 detection system permit to study the creation of mesons in the nucleon-nucleon interaction down to the fraction of MeV with respect to the kinematical threshold. At such small excess energies, the ejectiles possess low relative momenta and are predominantly produced with the relative angular momentum equal to zero. Taking advantage of these conditions we have performed investigations aiming to determine the mechanism of the production of η and η′ mesons in the collision of hadrons as well as the hadronic interaction of these mesons with nucleons and nuclei. In this proceedings we address the ongoing studies of the spin and isospin dependence for the production of the η and η′ mesons in free and quasi-free nucleon-nucleon collisions. New results on the spin observables for the $\vec{p}p\to pp\eta$ reaction, combined with the previously determined total cross section isospin dependence, reveal a statistically significant indication that the excitation of the nucleon to the S11(1535) resonance, the process which intermediates the production of the η meson in the nucleon-nuleon interactions, is predominantly due to the exchange of the π meson between the colliding nucleons. [ABSTRACT FROM AUTHOR]

Information about the production mechanism of the η meson in proton-proton collisions can be inferred by confronting the experimental studies on the analysing power for the $\vec{p}p\to pp\eta$ reaction with the theoretical predictions of this observable. The determined analysing powers for Q=10 MeV and Q=36 MeV are consistent with zero. Results show that the predictions of pure pseudoscalar meson exchange model fairly describe the experimental data, while the predictions of pure vector meson exchange dominance model disagree with the data at the level of 4.3 σ. [ABSTRACT FROM AUTHOR]

Preliminary results of recent measurements of the near threshold η meson production in the dp → dpη reaction are presented. The experiment was performed at the COSY-Juelich accelerator with the use of the COSY-11 detection system. Data were taken for three values of deuteron beam momenta corresponding to excess energies of 3.2, 6.1 and 9.2 MeV. The energy dependence of the total cross section confirms a strong effect of the final state interaction. [ABSTRACT FROM AUTHOR]

A brief experimental overview on the close-to-threshold η meson production in proton-proton interactions is presented and the available observables in measurements with unpolarized and polarized beam and target are discussed. [ABSTRACT FROM AUTHOR]

The n' -meson production in tne reaction pp → ppn' has been sturnea at excess energies o Q 26.5, 32.5 and 46.6 MeV using the internal beam facility COSY-11 at the cooler synchrotron COSY. The total cross-sections as well as one angular distribution for the highest Q-value are presented. The excitation function of the near-threshold data can be described by a pure s-wave phase space distribution with the inclusion of the proton-proton final-state interaction and Coulomb effects. The obtained angular distribution of the n'-mesons is also consistent with pure s-wave production. [ABSTRACT FROM AUTHOR]

First investigations on meson production at threshold in the strong interaction began in the fifties when sufficient energies of accelerated protons were available. Early experiments performed with bubble chambers revealed already typical ingredients of threshold studies, which were superseded by more complete meson production investigations at the nucleon beam facilities TRIUMF, LAMPF, PSI, LEAR and SATURNE. Currently, at the cooler rings IUCF, CELSIUS and COSY the field is entering a new domain of precision and further progress. The analysis of this new data in the short-range limit permits a more fundamental consideration and a quantitative comparison of the production processes for different mesons in the few-body final states. The interpretation of the data takes advantage of the fact that production reactions close to threshold are characterized by only a few degrees of freedom between a well-defined combination of initial and exit channels. Deviations from predictions of phase-space-controlled one-meson exchange models are indications of new physics. Precision data on differential cross-sections, isospin and spin observables --partly but by no means adequately available-- are presently turning up. [ABSTRACT FROM AUTHOR]

The production of η- and η′-mesons in elementary nucleon-nucleon collisions has been investigated at the synchrotrons CELSIUS, COSY and SATURNE. The high-quality proton beam with low emittance and small momentum spread permits to study the creation of these mesons very close to the kinematical threshold, where -due to the rapid growth of the phase space volume- the total cross-section increases by orders of magnitude over a few MeV range of the excess energy. The magnitude and energy dependence of the total cross-section as well as the occupation distribution of the phase space serve as observables for investigating the mechanisms underlying the production processes and the interaction of mesons with nucleons. The precise data on the η and η′ creation via the pp → ppη(η′) reactions allowed to settle the general features of the η and η′-meson production and revealed the sensitivity of the mentioned observables to the nucleon-nucleon-meson final-state interaction. The particular production properties, like for example, the determination of the dominating exchange processes which lead to the excitation of the S11 nucleon isobar in the case of η creation, must be established by confrontation with other observables. The present status of this investigation with an emphasis on the results of the COSY-11 Collaboration is briefly presented. The available data are interpreted in view of the production mechanism and the meson-nucleon interaction. [ABSTRACT FROM AUTHOR]

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 σ° production via the pp → pK+Λ/Σ° reaction (S. Sewerin et al., Phys. Rev. Lett. 83, 682 (1999)) at COSY-1 1 have shown that theΛΣ° cross-section ratio exceeds the value at high excess energies (Q &Ge; 300 MeV (A. Baldini et al., Total cross-Sections for Reactions of High-Energy Particles, Landolt-Börnstein, 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 fo(980) and ao(980) (0. Krehl, R. Rapp, J. Speth, Phys. Lett. B 390, 23 (1997)). The interpretation as a KK 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 → ppK+&Kmacr;- at an excess energy of 17MeV (C. Quentmeier et al., Phys. Lett. B 515, 276 (2001)), i.e. below the &phil; production threshold, was measured. [ABSTRACT FROM AUTHOR]

Simple–minded thoughts about the cross sections for the reactions pp → ppK+K- and pp → ppK0K0 are presented, which certainly do not account for the complex coupled channel problem but rather provide some ideas into the mutual reaction dynamics. [ABSTRACT FROM AUTHOR]

At the internal facility COSY-11 a first measurement of the reaction p′p → near the production threshold has been performed. Results for the analysing power will be presented and a comparison with one-meson exchange models will be discussed. [ABSTRACT FROM AUTHOR]

Measurements of the η meson production with a polarised proton beam in the reaction p→p→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. [Copyright &y& Elsevier]