New insights on $\nu$-DM Interactions

We revisit the possibility of using cosmological observations to constrain models that involve interactions between neutrinos and dark matter. We show that small-scale measurements of the cosmic microwave background with a few per cent accuracy are critical to uncover unique signatures from models with tiny couplings that would require a much higher sensitivity at lower multipoles, such as those probed by the Planck satellite. We analyze the high-multipole data released by the Atacama Cosmology Telescope, both independently and in combination with Planck and Baryon Acoustic Oscillation measurements, finding a compelling preference for a non-vanishing coupling, $\log_{10}u_{\nu \textrm{DM}}=-5.20^{+1.2}_{-0.74}$ at 68% CL. This aligns with other CMB-independent probes, such as Lyman-$\alpha$. We illustrate how this coupling could be accounted for in the presence of dark matter interactions with a sterile neutrino.