Confirmation of the existence of the X17 particle

In a 2016 paper, an anomaly in the internal pair creation on the M1 transition depopulating the 18.15 MeV isoscalar 1(+) state on Be-8 was observed. This could be explained by the creation and subsequent decay of a new boson, with mass m(X)c(2) = 16.70 MeV. Further experiments of the same transition with an improved and independent setup were performed, which constrained the mass of the X17 boson (m(X)c(2)) and its branching ratio relative to the gamma-decay of the Be-8 excited state (B-X), to m(X)c(2) = 17.01(16) MeV and B-X = 6(1) x 10(-6), respectively. Using the latter setup, the e(+)e(-) pairs depopulating the 21 MeV J(pi) = 0(-) -> 0(+) transition in He-4 were investigated and a resonance in the angular correlation of the pairs was observed, which could be explained by the same X17 particle, with mass m(X)c(2) = 16.98 +/- 0.16(stat) +/- 0.20(syst) MeV.