Discrepancies from in-beam- and in-bottle-type experiments measuring the neutron lifetime are on the $4\sigma$ standard deviation level. In a recent publication Fornal and Grinstein proposed that the puzzle could be solved if the neutron would decay on the one percent level via a dark decay mode, one possible branch being $n\to \chi +e^{+}{e}^{-}$. With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 95% of the allowed mass range for the dark matter particle.

• Received 13 November 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.222503

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Published by the American Physical Society