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, He-4, 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 x 10(10) 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 x 10(-4) less than or similar to epsilon(e) less than or similar to 6.8 x 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 less than or similar to 24 MeV. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling epsilon. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.