Connecting cosmologically decaying dark matter to neutrino physics

Dark matter decays into invisible particles can leave an imprint in large-scale structure surveys due to a characteristic redshift-dependent suppression of the power spectrum. We present a model with two quasi-degenerate singlet fermions, $\chi_1$ and $\chi_2$, in which the heavier state decays as $\chi_2 \to \bar \chi_1 \nu \nu$ on cosmological time-scales, and that also accommodates non-zero neutrino masses. Remarkably, for parameters that yield the correct dark matter abundance via freeze-in and reproduce the observed neutrino masses, dark matter decay can produce detectable signals in forthcoming large-scale structure surveys, a diffuse anti-neutrino flux accessible to JUNO, and a gamma-ray line within the energy range probed by COSI. Both the cosmological lifetime of $\chi_2$ as well as the small (radiatively induced) mass splitting among $\chi_{1,2}$ are a natural consequence of the mechanism of neutrino mass generation within this model. This highlights the potential role of large-scale structure surveys in probing some classes of neutrino mass models.