Eta production


Results of the study of the eta pp dynamics presented in the talk are avaiable via the e-Print Archive server [1]. Here we abridged the text to few remarks - from the introduction of this article - concerning investigations of the interaction between the eta meson and the proton.

Due to the short live time of the flavour-neutral mesons (eg. eta or eta'), the study of their interaction with nucleons or with other mesons is at present not feasible in direct scattering experiments. One of the methods permitting such investigations is the production of a meson in the nucleon-nucleon interaction close to the kinematical threshold or in kinematics regions where the outgoing particles possess small relative velocities. In the last decade major experimental [2-4] and theoretical [5,6] efforts were concentrated on the study of the creation of eta and eta' mesons via the hadronic interactions [7]. Measurements have been performed in the vicinity of the kinematical threshold where only a few partial waves in both initial and final state are expected to contribute to the production process.

The determined energy dependences of the total cross section for eta' [2,3] and eta [3,4] mesons in proton-proton collisions reveals that the proton-proton FSI enhances the total cross section by more than an order of magnitude for low excess energies. Interestingly, in the case of the eta meson the increase of the total cross section for very low and very high energies is much larger than expected from the 1S0 final state interaction between protons. The excess at higher energies can be assigned to the significant onset of higher partial waves, and the influence of the attractive interaction between the eta meson and the proton could be a plausible explanation for the enhancement at threshold.

The interaction between particles depends on their relative momenta. Therefore it should show up as modification of the phase-space abundance in the kinematical regions where the outgoing particles possess small relative velocities. Only twoinvariant masses of the three subsystems are independent and therefore the entire accessible information about the final state interaction of the three-particle system can be presented in the form of the Dalitz plot. To some extent this information is still available from the projections of the phase space population onto the invariant mass distributions. These have been recently determined at Q = 15 MeV by the COSY-TOF collaboration [8] and at Q = 15.5 MeV by the COSY-11 group [1]. The structure of the observed spectra may indicate a non-negligible contribution from the P-waves in the outgoing proton-proton subsystem [6]. The amount of the P-wave admixture derived from the proton-proton invariant mass distribution leads to a good description of the excitation function at higher excess energies while at the same time it spoils significanly the agreement with the data at low values of Q [6]. In contrast to the P-wave contribution the three-body treatment [9] of the pp eta system leads to an even larger enhancement of the cross section near threshold than that based on the Ansatz of the factorization of the proton-proton and proton-eta interactions. For the complete understanding of the low energy pp eta dynamics, a rigorous three-body approach to the pp eta system is required. A herald of such calculations have been already reported [9,10].

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[6] K. Nakayama et al., e-Print Archive: nucl-th/0302061.
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[8] M. Abdel-Bary et al., Eur. Phys. J. A 16, 127 (2003).
[9] A. Fix, H. Arenhövel, in preparation (2003).
[10] A. Deloff, e-Print Archive: nucl-th/0309059.cp

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