Possible ${\eta }^{\prime }d$ bound state and its $s$-channel formation in the $\gamma d\to \eta d$ reaction

Abstract

We theoretically investigate a possibility of an ${\eta }^{\prime }d$ bound state and its formation in the $\gamma d\to \eta d$ reaction. First, in the fixed center approximation to the Faddeev equations we obtain an ${\eta }^{\prime }d$ bound state with a binding energy of 25 MeV and width of 19 MeV, where we take the ${\eta }^{\prime }N$ interaction with a coupling to the $\eta N$ channel from the linear $\sigma$ model. Then, in order to investigate the feasibility from an experimental point of view, we calculate the cross section of the $\gamma d\to \eta d$ reaction at the photon energy in the laboratory frame around 1.2 GeV. As a result, we find a clear peak structure with the strength $\sim$ 0.2 nb/sr, corresponding to a signal of the ${\eta }^{\prime }d$ bound state in case of backward $\eta$ emission. This structure will be prominent because a background contribution coming from single-step $\eta$ emission off a bound nucleon is highly suppressed. In addition, the signal can be seen even in case of forward $\eta$ emission as a bump or dip, depending on the relative phase between the bound-state formation and the single-step background.

Comments: 9 pages, 13 eps figures, version accepted for publication in PRC, description of the discussion part is improved