Realistic model for a fifth force explaining anomaly in Be⁎8→Be8e+e− decay

We propose a theoretical model to explain a 6.8σ anomaly recently reported in the opening angle and invariant mass distributions of e+e− pairs produced in excited Be⁎8 nuclear transition to its ground state B8e. The anomaly is explained by a fifth force mediated by a 17 MeV X boson through the decay Be⁎8→Be8X followed by X→e+e−. The X boson comes from extension of the standard model with two additional U(1) gauge symmetries producing a protophobic pure vector current interaction with quarks. The model also contains axial-vector current interaction. Although the existent axial-vector current interactions are strongly constrained by the measurement of parity violation in e-quark scattering, their contributions cancel out in the iso-scalar interaction for Be⁎8→Be8X. It is remarkable that the model parameters need to explain the anomaly survive all known low energy experimental constraints. The model may also alleviate the long-standing (g−2)μ anomaly problem and can be probed by the LHCb experiment.