Ηn scattering parameters and possible η⁰d bound state from η photoproduction on the deuteron

Two physics programs, determination of low-energy scattering parameters between the eta meson (η) and nucleon (N), and search for a possible bound state between the eta prime meson (η⁰) and deuteron, using η photoproduction from the deuteron target are conducted at the Research Center for Electron Photon Science (ELPH), Tohoku University. Among the two-body dynamics of the meson–nucleon systems, the ηN interaction is not well-known although it has been found attractive. An experiment is carried out to determine the low-energy ηN scattering parameters using a special kinematics. The energy and momentum of the emitted proton (p) are measured at 0 degrees for η photoproduction on the deuteron (d) at incident energies around 0.94 GeV, which gives the low relative momentum between η and neutron (n) in the final state. Low-energy ηn scattering is likely to take place in this condition, and the scattering parameters can be determined from the differential cross section as a function of the ηn invariant mass. The measurement is currently in progress to determine the real part of the ηn scattering length. A possible η⁰d bound state is theoretically predicted, and a structure corresponding to the state can be observed via the γd → ηd reaction at incident energies around 1.2 GeV. In the case of backward η emission, the structure becomes prominent because a background contribution coming from quasi-free single-step η emission is highly suppressed. The γd → ηd reaction has been also studied at ELPH below the incident energy of 1.15 GeV. The angular differential cross sections are determined at backward η emission angles. The tail of the corresponding peak is not observed, and the background level is much higher than predicted.