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

Abstract

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.