Search for $\mathrm{NN}$-decoupled dibaryons using the process $p\overrightarrow{p}\gamma \gamma X$ below the pion production threshold

The energy spectrum for high energy $\gamma$ rays ${(E}_{\gamma }>~10$ MeV) from the process $p\overrightarrow{p}\gamma \gamma X$ emitted at $90°$ in the laboratory frame has been measured at an energy below the pion production threshold, namely, at 216 MeV. The resulting photon energy spectrum extracted from $\gamma -\gamma$ coincidence events consists of a narrow peak at a photon energy of about 24 MeV and a relatively broad peak in the energy range of $50-70$ MeV. The statistical significances for the narrow and broad peaks are $5.3\sigma$ and 3.5σ, respectively. This behavior of the photon energy spectrum is interpreted as a signature of the exotic dibaryon resonance $d_1^{\star }$ with a mass of about 1956 MeV, which is assumed to be formed in the radiative process $p\overrightarrow{p}\gamma d_1^{\star }$ followed by its electromagnetic decay via the $d_1^{\star }\to \mathrm{pp}\gamma$ mode. The experimental spectrum is compared with those obtained by means of Monte Carlo simulations.