A puzzling anomaly has been recently observed in the emission of electron-positron pairs in the 7Li(p,e+e-)8Be and 3H(p, e+e-)4He reactions. This anomaly has been interpreted as the signature of a
new boson particle, not foreseen in the Standard Model, with mass 16.8 MeV (hereafter called the X17 boson). Various theoretical speculations have been advanced, as for example that the X17 boson could be either a mediator of a fifth force, characterized by a strong coupling suppression of protons compared to neutrons, or a “piophobic” axion-like-particle. The fifth force scenario can explain, at least partially, the long standing anomaly of the magnetic moment of muon found experimentally. In order to test these assumptions, it is important to properly treat the nuclear dynamics. In this talk, we present the results of a theoretical study of the 3H(p, e+e-)4He and 3He(n, e+e-)4He reactions, and discuss how the different scenarios agree with the available experimental data. We will also briefly discuss about the present and future experimental activities stimulated to either confirm or reject this anomaly.