First close to threshold measurement of the analyzing power Ay in the reaction ppol ->pp eta at the experiment COSY-11


Experiments on meson production enable to understand the fundamental hadronic interaction and the underlying reaction mechanisms. Especially, the eta meson production has been studied by several experimental groups in the last years. It is well accepted that for this meson the production in nucleon-nucleon collisions is dominated by the intermediate virtual S11 nucleon isobar. Yet, measurements of the total and differential cross sections cannot disentangle between various scenarios of the S11 excitation. Predictions of the analyzing power depend crucially on the assumed mechanism [1, 2]. This fact triggered already experimental investigations on spin observables.

Using data measured at the detector system COSY-11, initial results of the analyzing power in the ppol ->pp eta reaction at a beam momentum of pbeam = 2.096 GeV/c - according to an excess energy Q = 40 MeV - are presented [3]. The selection of the pp eta final states succeeds with a kinematically complete reconstruction of the three particles in their final state. The determination of the relative luminosity for both spins is based on the measurement of the elastic proton-proton scattering. The time averaged beam polarisationwas extracted through the simultaneous measurement of the elastic pp-scattering at the experiment EDDA [4].

The analyzing power is studied for its dependence on the center of mass polar angle Thetaq of the eta meson and the relative kinetic energy Epp in the proton-proton rest system. Furthermore, an extraction of interference terms for different partial wave amplitudes is carried out. Finally, a comparison with current theoretical predictions is presented and discussed.

[1] G.Fäldt and C.Wilkin, Phys. Scripta 64 (2001) 427.
[2] K.Nakayama, J.Speth and T.S.Lee, e-Print Archive: nucl-th/0202012.
[3] P.Winter, Diploma work, Berichte des FZ Jülich; Jül-3943, 2002.
[4] M.Altmeier et al., Phys. Rev. Lett., 85 (2000) 1819 and F.Bauer, private communication.

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Eur.Phys.Jou. A 18 (2003) 355-357