The improved results on a direct search for a new $X(16.7\mathrm{MeV})$ boson that could explain the anomalous excess of $e^{+}{e}^{-}$ pairs observed in the decays of the excited ${}^8{\mathrm{Be}}^{*}$ nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, ${}^4\mathrm{He}$, seems to support this anomaly spurring the need for an independent measurement. If the $X$ boson exists, it could be produced in the bremsstrahlung reaction $e^{-}Z\to e^{-}ZX$ by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through its subsequent decay into $e^{+}{e}^{-}$ pairs. 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 one to set new limits on the $X-e^{-}$ coupling in the range $1.2\times {10}^{-4}\lesssim {\epsilon }_e\lesssim 6.8\times {10}^{-4}$, excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons ($A^{\prime }$) with a mass $\lesssim 24\mathrm{MeV}$. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling $\epsilon$. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.

• Received 20 January 2020
• Accepted 27 March 2020

DOI:https://doi.org/10.1103/PhysRevD.101.071101

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Published by the American Physical Society