Relativistic description of the np to eta d reaction near threshold
  Relativistic effects in a three-body calculation of the $np\to\eta d$ process are considered. Relativistic effects on the range and strength of the pion exchange contribution to the reaction mechanism may be large, but boost effects of the two-body interactions are negligible. The relativistic calculation confirms previous non-relativistic results, showing that the shape of the cross section near threshold is essentially determined by the $\eta d$ final-state interaction alone. As for the region away from threshold, the relativistic pion exchange contribution is seen to dominate the other mechanisms of the reaction. It turns out that, within the relativistic reaction model, the $np\to\eta d$ experimental data excludes large values for the real part of the $\eta N$ scattering length. 
H. Garcilazo M. T. Pena