Hints of neutrino dark matter scattering in the CMB? Constraints from the marginalized and profile distributions

We study scatterlike interactions between neutrinos and dark matter in light of different combinations of temperature, polarization and lensing data released by three independent CMB experiments-the Planck satellite, the Atacama Cosmology Telescope (ACT), and the South Pole Telescope (SPT)-in conjunction with baryon acoustic oscillation (BAO) measurements. We apply two different statistical methodologies. Alongside the usual marginalization technique, we cross-check all the results through a profile likelihood analysis. As a first step, working under the assumption of massless neutrinos, we perform a comprehensive (re)analysis aimed at assessing the validity of some recent results hinting at a mild preference for nonvanishing interactions from small-scale CMB data. We find compelling resilience in the results already documented in the literature, confirming that interactions with a strength u nu DM similar to 10-5-10-4 appear to be globally favored by ACT (both alone and in combination with Planck). This result is corroborated by the inclusion of additional data, such as the recent ACT-DR6 lensing likelihood, as well as by the profile likelihood analysis. Interestingly, a fully consistent preference for interactions emerges from SPT, as well, although it is weaker than the one obtained from ACT. As a second step, we repeat the same analysis considering neutrinos as massive particles. Despite the larger parameter space, all the hints pointing towards interactions are confirmed also in this more realistic case. In addition, we report a very mild preference for interactions in Planck thorn BAO alone (not found in the massless case) which aligns with small-scale data. While this latter result is not fully confirmed by the profile likelihood analysis, the profile distribution does confirm that interactions are not disfavored by Planck.