Search for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN
D. Banerjee (University of Illinois at Urbana Champaign, Urbana, 61801-3080 Illinois, USA); V. E. Burtsev (Tomsk State Pedagogical University, 634061 Tomsk, Russia); A. G. Chumakov (Tomsk State Pedagogical University, 634061 Tomsk, Russia); D. Cooke (ETH Zürich, Institute for Particle Physics and Astrophysics, CH-8093 Zürich, Switzerland); P. Crivelli (ETH Zürich, Institute for Particle Physics and Astrophysics, CH-8093 Zürich, Switzerland); et al - Show all 48 authors
We report the first results on a direct search for a new 16.7 MeV boson () which could explain the anomalous excess of pairs observed in the excited nucleus decays. Because of its coupling to electrons, the could be produced in the bremsstrahlung reaction by a 100 GeV beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a pair. With electrons on target, no evidence for such decays was found, allowing us to set first limits on the coupling in the range excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons () from nonobservation of the decay of the bremsstrahlung with a mass .