A few years ago we observed anomalous electron-positron angular correlations for the 18.15~MeV M1 transition of 8Be . This was interpreted as the creation and decay of an intermediate bosonic particle with a mass of m0c2=16.70(35)(stat)(50)(sys) MeV, which is now called X17. The possible relation of the X17 boson to the dark matter problem triggered an enormous interest in the wider physics community. We then re-investigated the 8Be anomaly with an improved, and independent setup, and confirmed the signal of the assumed X17 particle [2,3].
We also observed a similar anomaly in 4He , which could be described also by the creation and subsequent decay of the same X17 particle. Our results agree well with the present ab initio calculations of Viviani et al., .
Very recently, the 11B proton capture reaction was used for exciting the 17.2 MeV broad (Gamma= 1.15 MeV) resonance in 12C and studying their internal pair creation decay. Significant anomalies were observed in the angular correlation of the electron-positron pairs, at three different bombarding energies, which provides kinematic evidence for the X17 particle and supports their vector boson and fifth force explanation.
 A.J. Krasznahorkay et al., Phys. Rev. Lett. 116 (2016) 042501.
 A.J. Krasznahorkay et al., J. Phys.: Conf. Series 1056 (2018) 012028.
 A.J. Krasznahorkay et al., Acta Phys. Pol. B 50 (2019) 675.
 A.J. Krasznahorkay et al., Phys. Rev. C 104 (2021) 044003.
 M. Viviani et al., Phys. Rev. C 105, (2022) 014001.