Near Threshold Production of eta and eta' and Charged Kaon Pairs in Proton-Proton Collisions


The near threshold production of mesons in nucleon-nucleon collisions is of fundamental interest and provides a possibility to study various theoretical models. Close to threshold only few partial waves contribute and due to the low relative momenta of the ejectiles, final state interactions can be studied in more detail.
At the cooler synchrotron COSY various near threshold measurements have been performed using the internal installation COSY-11 [1]. In this contribution data for the eta, eta' and K+K- meson production in proton-proton collisions will be presented and discussed in view of possible production mechanisms.

1. The reactions pp->pp eta and pp->pp eta'

In case of the eta meson the production in NN interaction close to the production threshold has been studied in several theoretical works using one-boson exchange models and is assumed to proceed dominant through the excitation of the S11 nucleon resonance N*(1535) with a branching ratio N*(1535)->eta N of about 30-55 %. However, at energies close to the eta' threshold no such resonance is known. Thus, it is not expected that the eta' production via the excitation of baryonic resonances will be appreciable. To check different models data especially close to threshold are needed. Therefore, the production of eta and eta' mesons has been measured at COSY-11 at excess energies of Q=0-6.8 MeV (eta) and Q=1.5-25.0 MeV (eta'), respectively [2,3].

2. The reaction pp->K+K-

Nucleon-nucleon production cross sections of kaons and kaon pairs are of great interest for the understanding of many topics like the still controversially discussed structure of the f0(980) or medium effects of kaon production in nuclear matter. Different to the K+ meson, which can also be produced via hyperon resonances, up to now there exist no data for the production of K- mesons in the nucleon-nucleon scattering near threshold. Therefore, measurements of the elementary production process pp->K+K- have been investigated at COSY-11 at excess energies between Q=4.4 MeV and Q=56.1MeV [4,5].

S.Brauksiepe et al., Nucl. Instr. & Meth. A 376 (1996) 397
P.Wüstner, Ph.D. thesis, University Bochum (1998)
P.Moskal et al., Phys. Rev. Lett. 80 (1998) 3202
T.Lister, Ph.D. thesis, University Münster (1998)
M.Wolke, Ph.D. thesis, University Bonn (1998)

Abstract in PostScript format (41kB)
Complete document in PostScript format (122kB)
Publication: Nucl.Phys. A 663-664 (2000) 565-568