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

Discrepancies from beam and bottle type experiments measuring the neutron lifetime are on the 4*σ 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, 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. (authors)