Search for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN

We report the first results on a direct search for a new 16.7 MeV boson (X) which could explain the anomalous excess of e⁺e⁻ pairs observed in the excited ⁸Be* nucleus decays. Because of its coupling to electrons, the X could be produced in the bremsstrahlung reaction e⁻Z→e⁻ZX by a 100 GeV e⁻ beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a e⁺e⁻ pair. With 5.4×10¹⁰ electrons on target, no evidence for such decays was found, allowing us to set first limits on the X-e⁻ coupling in the range 1.3×10⁻⁴≲εₑ≲4.2×10⁻⁴ excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons (A') from nonobservation of the decay A'→e⁺e⁻ of the bremsstrahlung A' with a mass ≲23 MeV.