$eta N$ Scattering Parameters and Possible $eta 'd$ Bound State from $eta $ Photoproduction on the Deuteron

Two physics programs, determination of low-energy scattering parameters between the eta meson (eta) and nucleon (N), and search for a possible bound state between the eta prime meson (eta') and deuteron, using eta 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 eta N interaction is not well-known although it has been found attractive. An experiment is carried out to determine the low-energy eta N scattering parameters using a special kinematics. The energy and momentum of the emitted proton (p) are measured at 0 degrees for eta photoproduction on the deuteron (d) at incident energies around 0.94 GeV, which gives the low relative momentum between eta and neutron (n) in the final state. Low-energy eta n 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 ran invariant mass. The measurement is currently in progress to determine the real part of the eta n scattering length. A possible eta'd bound state is theoretically predicted, and a structure corresponding to the state can be observed via the gamma d -> eta d reaction at incident energies around 1.2 GeV. In the case of backward eta emission, the structure becomes prominent because a background contribution coming from quasi-free single-step n emission is highly suppressed. The gamma d -> eta d reaction has been also studied at ELPH below the incident energy of 1.15 GeV. The angular differential cross sections are determined at backward eta emission angles. The tail of the corresponding peak is not observed, and the background level is much higher than predicted.