Np → ηd near threshold and the s-wave ηn amplitude

We calculate the process np → ηd near threshold using a separable potential model of the coupled ηN - πN - ππN subystems, and a relativistic three-body calculation for the ηd scattering amplitude. The ππN channels are represented by an effective σN channel, and we compare the case where the σ and π masses are related by m _σ = 2m_π and no width is considered, to another where the mass and width of the σ-meson are taken from ππ scattering data. The np → ηd cross-section can be well described up to about 60MeV by models where the real part of the ηN scattering length lies between 0.4 ≤ Re(a_ηN) ≤ 0.6 fm which allows us to determine the s-wave ηN scattering amplitude for -60 ≤ E ≤ 60 MeV.