Properties of η-meson production in nucleon-nucleon collisions near threshold

Polarization observables in η production induced by NN collisions p+p→p+p+η, n+p→n+p+η, and n+p→d+η, are derived. The unique S wave spin structure of the threshold amplitude for p+p→p+p+η allows us to calculate all polarization observables in an universal form. Asymmetries in n→p→ collisions, characterized by noninterfering singlet and triplet amplitudes, are calculated. From the large experimental ratio of the differential cross sections of the processes n+p→n+p+η and p+p→p+p+η, near threshold, one predicts large spin correlation coefficients C_xx (C_yy). The spin structure of S- and P- wave contributions is derived.