Employing the ${}^{11}\mathrm{B}(p,\gamma ){}^{12}\mathrm{C}$ nuclear reaction, the angular correlation of $e^{+}{e}^{-}$ pairs was investigated in the angular range of ${40}^{\circ }\le \mathrm{\Theta }\le {175}^{\circ }$ for five different proton energies between $E_p=1.50$–2.5 MeV. At small angles ($\mathrm{\Theta }\le {120}^{\circ }$), the results can be well interpreted by the internal pair creation process of electromagnetic radiations with $E1$ and $M1$ multipolarities and by the external pair creation in the target backing. However, at angles greater than ${120}^{\circ }$, additional count excesses and anomalies were observed, which could be well accounted for by the existence of the previously suggested hypothetical X17 particle. Our results suggest that the X17 particle was generated mainly in $E1$ radiation. The derived mass of the particle is $m_{\mathrm{X}}{c}^2=17.03\pm 0.11\left(\mathrm{stat}\right)\pm 0.20\left(\mathrm{syst}\right)$ MeV. According to the mass, and to the derived branching ratio [$B_x=3.6\left(3\right)\times {10}^{-6}$], this is likely the same X17 particle that we recently suggested for describing the anomaly observed in the decay of ${}^8\mathrm{Be}$ and ${}^4\mathrm{He}$.

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• Nuclear tests of fundamental interactions