The existence of an I=0, ${\mathit{J}}^{\mathit{P}}$=$1^{\mathrm{-}}$ quasibound state in the ηNN-πNN coupled system is theoretically predicted with a mass of about 2430 MeV and a width 10–20 MeV. The three-body equation for the ηNN-πNN coupled system. The primary two-body interactions generating the bound state are (1) the NN interaction in the ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$${\mathrm{-}}^3$${\mathit{D}}_1$ state and (2) the πN and ηN coupled interaction in the ${\mathit{S}}_{11}$ state. A remarkable enhancement of the elastic cross section of η-d scattering near the ηd threshold is found, whose origin is in the pole structure of the I=0, ${\mathit{J}}^{\mathit{P}}$=$1^{\mathrm{-}}$ ηd scattering amplitude on the complex energy plane.

• Received 24 September 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.297