Scalar-meson production in nucleon-nucleon collisions near threshold

We establish the model-independent spin structure of the matrix elements for the near-threshold scalar-meson production in pp and np collisions, when the final particles are emitted in S-state. Polarization phenomena are derived in a general form. The properties of the t-channel dynamics, which is based on different meson exchanges, are studied in terms of the s-channel parametrization of the matrix element. The predictions of a model, based on $\pi + \sigma$ exchanges are also presented