The X17 anomaly


A rather 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 particle not foreseen in the standard model of particle physics (hereafter X17 boson) with mass M=16.8 MeV. The X17 boson could be a mediator of a fifth force, characterized by a strong coupling suppression of protons compared to neutrons. In this talk, we present an ab-initio study of the 3H(p, e+e-)4He and 3He(n,e+e-)4He processes. We first analyze the pair production as a purely electromagnetic process in the context of a state-of-the-art approach to nuclear strong-interaction dynamics and nuclear electromagnetic currents, derived from chiral effective field theory (chiEFT).  Next, we examine how the exchange of a hypothetical low-mass boson would impact the cross section for such a process. We consider several possibilities, that this boson is either a scalar, pseudoscalar, vector, or axial particle. We also provide an overview of an experiment probing pair production in the 3He(n,e+e-)4He at the n_ToF facility at CERN, currently under construction. We discuss also of other experimental searches of the  17 boson in nuclear reactions, as that one performed by the MEG collaboration at PSI (Switzerland), which are currently repeating the 7Li(p,e+e-)8Be experiment, or as in the 2H(p,e+e-)3He and 2H(n,e+e-)3H reactions, proposed in order to test the “protophobicity” of X17.


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