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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 excitedBe-8 nucleus decays. Due to its coupling to electrons, the X could be producedin the bremsstrahlung reaction e- Z -> e- Z X by a 100 GeV e- beam incident onan active target in the NA64 experiment at the CERN SPS and observed throughthe subsequent decay into an e+e- pair. With 5.4\times 10^{10} electrons ontarget, no evidence for such decays was found, allowing to set first limits onthe X-e^- coupling in the range 1.3\times 10^{-4} < \epsilon_e < 4.2\times10^{-4} excluding part of the allowed parameter space. We also set new boundson the mixing strength of photons with dark photons (A') from non-observationof the decay A'->e+e- of the bremsstrahlung A' with a mass <~ 23 MeV.

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