Improved limits on a hypothetical X(16.7) boson and a dark photon decaying into $e^+e^-$ pairs

TL;DR

This paper reports improved experimental limits on a hypothetical X(16.7) boson and dark photon decaying into electron-positron pairs, constraining their possible couplings and excluding parts of the parameter space suggested by previous anomalies.

Contribution

The study provides new, more stringent bounds on the X(16.7) boson and dark photon parameters using high-statistics electron beam data from the NA64 experiment.

Findings

No evidence for X(16.7) boson decays was observed.

New limits on the X-e coupling were established, excluding some favored parameter regions.

Bounds on dark photon mixing strength were improved for masses below 24 MeV.

Abstract

The improved results on a direct search for a new X(16.7 MeV) boson which could explain the anomalous excess of $e^+e^-$ pairs observed in the excited 8Be nucleus decays ("Berillium anomaly") are reported. Due to its coupling to electrons, the X boson could be produced in the bremsstrahlung reaction e-Z -> e-ZX by a high-energy beam of electrons incident on active target in the NA64 experiment at the CERN SPS and observed through its subsequent decay into $e^+e^-$ pair. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to 8.4\times 10^{10} electrons on target collected in 2017 and 2018. This allows to set the new limits on the $X$--$e^-$ coupling in the range 1.2 \times 10^{-4} < \epsilon_e < 6.8 \times 10^{-4}, excluding part of the parameter space favored by the Berillium anomaly. We also set new bounds on the mixing strength of photons with dark photons (A') from non-observation of the decay $A' \to e^+e^-$ of the bremsstrahlung A' with a mass below 24 MeV.