Existence of exotic neutron decays into nearly mass-degenerate long-lived particles

Determine whether free neutrons possess rare exotic decay modes into nearly mass-degenerate long-lived neutral particles (for example, n → X + γ with X having a lifetime consistent with current bounds), by conducting targeted searches at HIBEAM/NNBAR that can detect such decays or place stringent limits on their occurrence.

Background

This section proposes searches at ESS/HIBEAM–NNBAR for exotic neutron decay channels producing long-lived particles X with mass close to the neutron’s mass. The authors discuss the allowed parameter space given constraints from underground detectors, outline plausible signatures (photons, charged pions, missing energy), and estimate potential event yields under realistic beam conditions.

The passage explicitly labels the existence of such exotic decays as a fundamental open question and motivates low-cost parasitic searches using existing infrastructure to address it.