A Covariant OBE Model for $\eta$ Production in NN Collisions

A relativistic covariant one boson exchange model, previously applied to describe elastic nucleon-nucleon scattering, is extended to study $\eta$ production in NN collisions. The transition amplitude for the elementary BN->$\eta$N process with B being the meson exchanged (B=$\pi$, $|sigma$,$\eta$, $\rho$, $\omega$ and $\delta$) are taken to be the sum of four terms corresponding to s and u-channels with a nucleon or a nucleon isobar N*(1535MeV) in the intermediate states. Taking the relative phases of the various exchange amplitudes to be +1, the model reproduces the cross sections for the $NN\to X\eta$ reactions in a consistent manner. In the limit where all $\eta$'s are produced via N^* excitations, interference terms between the overall contributions from the exchange of pseudoscalart and scalar mesons with that of vector mesons cancel out. Consequently, much of the ambiguities in the model predictions due to unknown relative phases of different vector pseudoscalar exchanges are strongly reduced.