Dark decay channel analysis (n → χ + e⁺ e⁻) with the PERKEO II experiment

Abstract

Discrepancies from beam and bottle type experiments measuring the neutron lifetime are on the 4σ level. In recent publications 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 [1], one possible branch being n → χ + e+e−. With data from the Perkeo II experiment we set limits on the branching fraction and exclude a one percent contribution for 96% of the allowed mass range for the dark matter particle. With this publication, we give a detailed description of the experiment and some selected details of the analysis.