New anomaly observed in ${}^{12}\mathrm{C}$ supports the existence and the vector character of the hypothetical X17 boson

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 ${}^4\mathrm{He}$.

Figure 1

(a) Detector response for the setup as a function of correlation angle ($\theta$) for isotropic emission of $e^{+}{e}^{-}$ pairs (red crosses) compared with the results of the Monte Carlo simulations (black line histogram) as explained in the text. b) $e^{+}{e}^{-}$ angular correlations obtained for the 17.6 MeV transition of ${}^8\mathrm{Be}$ by using thin target backing compared to the simulations performed for $E1$ and $M1$ IPC, as well as for the EPC created by the $\gamma$-rays on the different materials around the target.

Figure 2

Total energy spectrum of the $e^{+}{e}^{-}$-pairs from the ${}^{11}\mathrm{B}(p,e^{+}{e}^{-}){}^{12}\mathrm{C}$ nuclear reaction.

Figure 3

(a) Experimental angular correlations of the $e^{+}{e}^{-}$ pairs measured in the ${}^{11}\mathrm{B}(p,e^{+}{e}^{-}){}^{12}\mathrm{C}$ reaction at the vicinity of the $J^{\pi }=1^{+}$ resonance for different proton energies.

Figure 4

Experimental angular correlations of the $e^{+}{e}^{-}$ pairs measured at different proton energies. The full curves for each proton energy shows the results of the fit, using simulated angular distributions.