On the structure in the $\Lambda N$ cross section at the $\Sigma N$ threshold

The complexity of threshold phenomena is exemplified on a prominent and long-known case - the structure in the $\Lambda p$ cross section (invariant mass spectrum) at the opening of the $\Sigma N$ channel. The mass splitting between the $\Sigma$ baryons together with the angular momentum coupling in the $^3S_1$-$^3D_1$ partial wave imply that, in principle, up to six channels are involved. Utilizing hyperon-nucleon potentials that provide an excellent description of the available low-energy $\Lambda p$ and $\Sigma N$ scattering data, the shape of the resulting $\Lambda p$ cross section is discussed and the poles near the $\Sigma N$ threshold are determined. Evidence for a strangeness $S=-1$ dibaryon is provided, in the form of a deuteron-like (unstable) $\Sigma N$ bound state. Predictions for level shifts and widths of $\Sigma^-p$ atomic states are given.