Studies of meson production in nucleon-nucleon collisions at threshold are characterised by few degrees of freedom in a configuration of well defined initial and final states coupled by short range dynamics. Effects from low-energy scattering in the exit channel are inherent to the data and probe the interaction in meson-nucleon and meson-meson systems otherwise difficult to access. From dedicated experiments at the present generation of cooler rings precise data are becoming available on differential and eventually spin observables allowing detailed comparisons between different final states. To discuss physics implications of generic and specific properties, recent experimental results on meson production obtained at CELSIUS and COSY serve as a guideline:
Exclusive data on pi+pi- production in proton-proton collisions suggest the reaction channel to be determined close to threshold by sigma exchange excitation and subsequent decay of the Roper resonance N*(1440). Consequently, the data probe the N pi pi resonance decay mode, and experiments at CELSIUS give first evidence for a different energy dependence of competing decay routes [1].
Likewise, predominant resonant contributions for eta production via an excitation of the N*(1535) are well established. However, considerable uncertainty remains with respect to the relative contribution of different meson exchanges. Results on differential observables are available from high statistics eta production: A possible onset of higher partial waves close to threshold is vividly discussed at present stimulated by some features of the data [2]. Spin observables are predicted to differ considerably depending on the underlying reaction mechanism, and have become accessible from the development of the polarized proton beam at COSY [3].
In elementary hadronic interactions with no strange valence quark in the initial state the associated strangeness production provides a powerful tool to study reaction dynamics by introducing a tracer to hadronic matter. Data on open strangeness production via both the elementary kaon and antikaon channels are available from exclusive experiments at COSY: First data on kaon-hyperon production showed an enhancement by one order of magnitude of the Lambda / Sigma0 ratio at equal excess energies close to threshold compared to high energy results. The transition regime has been studied and the new data exhibit qualitative differences between both hyperon production channels [4].
[1] J. Pätzold et al., e-Print arXiv: nucl-ex/0301019.
[2] P. Moskal et al., e-Print arXiv: nucl-ex/0212003;
M. Abdel-Bary et al., Eur. Phys. J. A 16 (2003) 127;
K. Nakayama et al., e-Print arXiv: nucl-ex/0302061.
[3] P. Winter et al., Phys. Lett. B 544 (2002) 251;
Erratum-ibid. B 553 (2003) 339.
[4] S. Sewerin et al., Phys. Rev. Lett. 83 (1999) 682;
P. Kowina, Ph.D. thesis, Silesian University Katowice (2002).
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