Search for NN-decoupled dibaryons using the process pp → γγX below the pion production threshold

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The energy spectrum for high energy γ-rays (Eγ≥ 10 MeV) from the process pp → γγX emitted at 90⁰ 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 5.3σ and 3.5σ, respectively. This behavior of the photon energy spectrum is interpreted as a signature of the exotic dibaryon resonance d⋆₁ with a mass of about 1956 MeV which is assumed to be formed in the radiative process pp → γd⋆₁ followed by its electromagnetic decay via the d⋆₁ → pp γ mode. The experimental spectrum is compared with those obtained by means of Monte Carlo simulations.