Improved limits on a hypothetical X(16.7) boson and a dark photon decaying into $e^+e^-$ pairs

The improved results on a direct search for a new X(16.7 MeV) boson that could explain the anomalous excess of e+e- pairs observed in the decays of the excited Be*8 nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, He4, seems to support this anomaly spurring the need for an independent measurement. If the X boson exists, it could be produced in the bremsstrahlung reaction e-Z→e-ZX by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through its subsequent decay into e+e- pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4×1010 electrons on target collected in 2017 and 2018. This allows one to set new limits on the X-e- coupling in the range 1.2×10-4≲εe≲6.8×10-4, excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons (A′) with a mass ≲24 MeV. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling ε. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.