Abstract
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 .
- Received 12 March 2018
DOI:https://doi.org/10.1103/PhysRevLett.120.231802
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Published by the American Physical Society