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.