New evidence supporting the existence of the hypothetic X17 particle

Abstract

We observed electron-positron pairs from the electro-magnetically forbidden M0 transition depopulating the 21.01 MeV 0${}^{-}$ state in ${}^4$He. A peak was observed in their $e^{+}{e}^{-}$ angular correlations at 115${}^{\circ }$ with 7.2$\ddot{I}ƒ$ significance, and could be described by assuming the creation and subsequent decay of a light particle with mass of $m_{\mathrm{X}}{c}^2$=16.84$\pm 0.16(stat)\pm 0.20(syst)$ MeV and $\hat{I}{“}_{\mathrm{X}}$= $3.9\times 10^{-5}$ eV. According to the mass, it is likely the same X17 particle, which we recently suggested [Phys. Rev. Lett. 116, 052501 (2016)] for describing the anomaly observed in ${}^8$Be.

5 pages, 4 figures