Confirmation of the existence of the X17 particle

In a 2016 paper, an anomaly in the internal pair creation on theM1transition 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 mXc2=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 (mXc2) and its branching ratio relative to the γ-decay of the 8Be excited state (BX), to mXc2=17.01(16) MeV and BX=6(1)×10−6, respectively. Using the latter setup, the e+e− pairs depopulating the 21 MeV Jπ=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 mXc2=16.98±0.16(stat)±0.20(syst) MeV