Search for NN-decoupled dibaryons using the process $pp\to \gamma \gamma X$ below the pion production threshold

Abstract

The energy spectrum for high energy $\gamma$-rays ($E_{\gamma }\ge 10$ MeV) from the process $pp\to \gamma \gamma X$ 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 $\gamma -\gamma$ 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 3.5$\sigma$, 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 $pp\to \gamma d_1^{\star }$ followed by its electromagnetic decay via the $d_1^{\star }\to pp\gamma$ mode. The experimental spectrum is compared with those obtained by means of Monte Carlo simulations.

Comments: 14 pages, LaTex, 6 eps-figures, accepted for publication in Phys.Rev.C