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

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} < ε_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 $A^{\prime }\to e^{+}{e}^{-}$ of the bremsstrahlung A' with a mass below 24 MeV.

7 pages, 5 figures, 1 table. arXiv admin note: substantial text overlap with arXiv:1803.07748