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

## TL;DR

This paper reports the first direct search for a hypothetical 16.7 MeV gauge boson and dark photons at CERN, setting new limits on their couplings based on null results from electron beam experiments.

## Contribution

It provides the first experimental limits on the 16.7 MeV boson and dark photon couplings using high-energy electron beam data at CERN.

## Key findings

- No evidence for the 16.7 MeV boson was observed.
- New bounds on the X-e^- coupling were established.
- Constraints on dark photon mixing with photons were improved.

## 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 Be-8 nucleus decays. Due to 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 SPS and observed through the subsequent decay into an e+e- pair. With 5.4\times 10^{10} electrons on target, no evidence for such decays was found, allowing to set first limits on the X-e^- coupling in the range 1.3\times 10^{-4} < \epsilon_e < 4.2\times 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 non-observation of the decay A'->e+e- of the bremsstrahlung A' with a mass <~ 23 MeV.