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

Recently, we observed an anomalous peak-like excess of internal e(+)e(-) pairs at around 140 degrees for the M1 transition depopulating the 18.15 MeV isoscalar 1(+) state in Be-8. 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 Be-8 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 He-4, and observed an anomalous excess of e(+)e(-) pairs at a significantly smaller angle of 115 degrees. 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.