Search for η′ Bound Nuclei in the C 12 (γ,p) Reaction with Simultaneous Detection of Decay Products

We measured missing mass spectrum of the C12(γ,p) reaction for the first time in coincidence with potential decay products from η′ bound nuclei. We tagged an (η+p) pair associated with the η′N→ηN process in a nucleus. After applying kinematical selections to reduce backgrounds, no signal events were observed in the bound-state region. An upper limit of the signal cross section in the opening angle cosθlabηp<-0.9 was obtained to be 2.2 nb/sr at the 90% confidence level. It is compared with theoretical cross sections, whose normalization ambiguity is suppressed by measuring a quasifree η′ production rate. Our results indicate a small branching fraction of the η′N→ηN process and/or a shallow η′-nucleus potential.