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 8Be was observed. This could be explained by the creation and subsequent decay of a new boson, with mass m X c2 = 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 c2) and its branching ratio relative to the γ-decay of the 8Be excited state (B X ), to m X c2 = 17.01(16) MeV and B X = 6(1) 10−6, respectively. Using the latter setup, the e +e − pairs depopulating the 21 MeV J π = 0− 0+ transition in 4He 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 c2 = 16.98 ± 0.16(stat) ± 0.20(syst) MeV.