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

Abstract

Employing the $^{11}$B(p,$\gamma$)$^{12}$C nuclear reaction, the angular correlation of $e^+e^-$ pairs was investigated in the angular range of $40^\circ\Theta\leq 175^\circ$ for five different proton energies between E$_p$ = 1.50 - 2.50 MeV. At small angles ($\Theta\leq 120^\circ$), 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 excess and anomalies were observed, which could be well accounted for by the existence of the previously suggested hypotetical X17 particle. Our results show that the X17 particle was generated mainly in E1 radiation. The derived mass of the particle is $m_\mathrm{X}c^2$=17.03$\pm 0.11 (stat) \pm 0.20 (syst)$ MeV. According to the mass, and to the derived branching ratio ($B_x=3.6(3)\times10^{-6}$), this is likely the same X17 particle, which we recently suggested for describing the anomaly observed in the decay of $^8$Be and $^4$He.