Search for NN-decoupled dibaryons using the process $pp \to γγX$ below the pion production threshold
The energy spectrum for high energy $γ$-rays ($E_γ\geq 10$ MeV) from the process $pp \to γγX$ emitted at $90^0$ 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 $γ-γ$ 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$σ$ and 3.5$σ$, respectively. This behavior of the photon energy spectrum is interpreted as a signature of the exotic dibaryon resonance $d^\star_1$ with a mass of about 1956 MeV which is assumed to be formed in the radiative process $pp \to γd^\star_1$ followed by its electromagnetic decay via the $d^\star_1 \to pp γ$ mode. The experimental spectrum is compared with those obtained by means of Monte Carlo simulations.