Neutrino mass limits and decaying dark matter: background evolution versus perturbations.

We revisit cosmological neutrino mass bounds when a fraction of dark matter decays into massless dark radiation. This decaying dark matter (DDM) can make datasets insensitive to neutrino masses at the background level. However, including perturbation observables like CMB lensing breaks this degeneracy, restoring strong constraints on neutrino mass in DDM scenarios. Our results emphasize the critical role of structure-growth measurements for robust cosmological bounds.