  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. Lett. 116, 052501 (2016)] for describing the anomaly observed in
$^8$Be.
