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 <mml:mmultiscripts>Be^* 8 </mml:mmultiscripts> nucleus decays. Because of its coupling to electrons, the X could be produced in the bremsstrahlung reaction e^-Z →e^-Z X 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≲ɛ_e≲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 .