Production of eta and eta' mesons via the quasi-free proton-neutron interaction


A comparison of the close-to-threshold total cross sections for the eta' meson production in both the pp->pp eta' and pn->pn eta' reactions should provide insight into the gluonium content of eta' meson and the relevance of quark-gluon or hadronic degrees of freedom in the creation process. Additionally to the mechanisms associated with meson exchanges it is possible that the eta' meson is created from the glue which is excited in the interaction region of the colliding nucleons [1], and which could couple to the eta' meson directly via its gluonic component or through its SU(3)-flavour-singlet admixture. The production through the colour-singlet object as suggested in reference [1] is isospin independent and hence should lead to the same production yield of the eta' meson in the pn->pn eta' and pp->pp eta' reactions after correcting for the final and initial state interaction between the nucleons.

Investigations of the eta-meson production in nucleon collisions allowed to conclude that close to the kinematical threshold the creation of eta meson with total isospin I = 0 exceeds the production with I = 1 by about a factor of 12. This was derived from the measured ratio of the total cross sections for the reactions pn->pn eta and pp->pp eta (Reta = sigma(pn->pn eta)nbsp;/nbsp;sigma(pp->pp eta)), which was determined to be Reta ~ 6.5 in the excess energy range between 16 MeV and 109 MeV [2].

Since the quark structure of eta and eta' mesons is very similar we can - by analogy to the eta meson production - expect that in the case of dominant isovector meson exchange the ratio Reta' should be about 6.5. If, however, the eta' meson was produced via its flavour-blind gluonic component from the colour-singlet glue excited in the interaction region, the ratio Reta' should approach unity after corrections for the interactions between the participating baryons.

So far we have determined the close-to-threshold excitation function for the pp->pp eta' reaction [3,4] whereas the total cross section for the eta' meson production in the proton-neutron interaction remains unknown. In this report first results of the measurements of the quasi-free pn->pn eta' reaction performed using the newly extended COSY-11 facility [5] will be presented.

[1] S. Bass, Phys. Lett. B 463 (1999) 286.
[2] H. Calén et al., Phys. Rev. C 58 (1998) 2667.
[3] COSY-11: P.Moskal et al., Phys. Rev. Lett. 80 (1998) 3202.
[4] COSY-11: P .Moskal et al., Phys. Lett. 8B 474 (2000) 416.
[5] P. Moskal, Schriften des FZ-Jülich: Matter & Material 11 (2002) 27.

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AIP Conf.Proc. 717 (2004) 907-911