Michele Viviani (INFN Pisa)

A spectacular 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 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 that could be an axion-like-particle. This scenario could explain, at least partially, the long standing anomaly of the magnetic moment of muon found experimentally. In order to test these hypothesis, 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 also present the current experimental activities trying to solve this puzzle, as, for example, that being developed by the n_TOF collaboration at CERN.