Search for $\eta'$ bound nuclei in the $^{12}{\rm C}(\gamma,p)$ reaction with simultaneous detection of decay products

Phys. Rev. Lett. 124, 202501 (2020) We measured missing mass spectrum of the $^{12}{\rm C}(\gamma,p)$ reaction for the first time in coincidence with potential decay products from $\eta'$ bound nuclei. We tagged an ($\eta+p$) pair associated with the $\eta'N\to\eta N$ 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 $\cos\theta^{\eta p}_{lab}