New evidence supporting the existence of the hypothetic X17 particle

Understand

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$\sigma$ 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$\pm0.16 (stat) \pm 0.20 (syst)$ MeV and $\Gamma_\mathrm{X}$= $3.9\times 10^{-5}$ eV.
• According to the mass, it is likely the same X17 particle, which we recently suggested [Phys.
• Rev.