Joint 21-cm and CMB forecasts for constraining self-interacting massive neutrinos

Self-interacting neutrinos provide an intriguing extension to the Standard Model, motivated by both particle physics and cosmology. Recent cosmological analyses suggest a bimodal posterior for the coupling strength G_eff, favoring either strong or moderate interactions. These interactions modify the scale dependence of the growth of cosmic structures, leaving distinct imprints on the matter power spectrum at small scales, k > 0.1 Mpc⁻¹. For the first time, we explore how the 21-cm power spectrum from the cosmic dawn and the dark ages can constrain the properties of self-interacting massive neutrinos. The effects of small-scale suppression and enhancement in the matter power spectrum caused by self-interacting neutrinos propagate to the halo mass function, shaping the abundance of small- and intermediate-mass halos. It is precisely these halos that host the galaxies responsible for driving the evolution of the 21-cm signal during the cosmic dawn. We find that Hydrogen Epoch of Reionization Array (HERA) at its design sensitivity can improve upon existing constraints on G_eff and be sensitive to small values of the coupling, beyond the reach of current and future cosmic microwave background (CMB) experiments. Crucially, we find that the combination of HERA and CMB-Stage 4 can break parameter degeneracies, significantly improving the sensitivity to G_eff over either experiment alone. Finally, we investigate the prospects of probing neutrino properties with futuristic Lunar interferometers, accessing the astrophysics-free 21-cm power spectrum during the dark ages. The capability of probing small scales of these instruments will allow us to reach a percent-level constraint on the neutrino self-coupling.