Recently, we used the 7Li(p,e+e-)8Be reaction to excite an 18.15 MeV excited state in 8Be and observed its internal pair (e+e-) decay to the ground state. An anomaly in the form of peak-like enhancement relative to the internal pair creation was observed at large angles in the angular correlation [1]. It turned out that this could be a first hint for a new mXc2=17 MeV X-boson, called X17 in the literature. Zhang and Miller discussed in detail whether a possible explanation of nuclear physics origin could be found [3]. They have not found any of such explanation.

Using a significantly modified and improved experimental setup, we reinvestigated the anomaly observed in the e+e- angular correlation by using a new tandetron accelerator of our institute. This setup has different efficiency curve as a function of the correlation angle, and different sensitivity to cosmic rays yielding practically independent experimental results. In this experiment, the previous data were reproduced within the error bars.

To confirm the existence of the X17 boson, we conducted a search for similar anomalies in another nuclear transitions. I am going to speak about our most recent results for the the 0  0+ transition in 4He, which energy is 21.1 MeV. We observed e+e- pairs with an angular correlation characteristic basically to the external pair creation (EPC) of the γ-rays created in the direct capture process of the 3H(p,γ)4He reaction and some contribution from the 0+  0+ E0 process. However, on top of the smooth background a peak at Θ≈115° is clearly visible with larger than 7σ confidence. According to our simulations performed with GEANT4, this peak corresponds to the decay of the X17 boson created in the 0 - 0+ transition.

[1] A.J. Krasznahorkay et al., Phys. Rev. Lett. 116, 042501 (2016)
[2] J. Feng et al., Phys. Rev. Lett. 117, 071803 (2016)
[3] Xilin Zhang and Gerald A. Miller, Phys. Lett. B773, 159 (2017)
[4] Ulrich Ellwanger and Stefano Moretti, JHEP 11, 039 (2016)
[5] Jonathan Kozaczuk, David E. Morrissey, and S. R. Stroberg, Phys. Rev. D 95, 115024
(2017)