Possible η′d bound state and its s -channel formation in the γd→ηd reaction

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