A new anomaly observed in $^4$He supporting the existence of the hypothetical X17 particle

Recently, we observed an anomalous peak-like excess of internal $e^+e^-$ pairs at around 140° for the M1 transition depopulating the 18.15 MeV isoscalar $1^+$ state in $^8$Be. The deviation from the theoretical prediction can be described by GEANT simulations assuming the creation and subsequent decay of a new, light boson with a mass of 16.7 MeV/$c^2$. In order to reduce the possible systematic errors from the experimenntal data, we re-investigated the $^8$Be anomaly with an improved setup and confirmed the anomaly within the statistical uncertainties. We also studied the angular correlation of the electron-positron pairs created in the M0 transition depopulating the 21.01 MeV 0$^-$ state in $^4$He, and observed an anomalous excess of $e^+e^-$ pairs at a significantly smaller angle of 115°. Since the transition energy was higher in this case, the observed anomaly could be described by assuming the creation and subsequent decay of the same light particle in the simulations.