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.

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.