$S_8$ Tension in the Context of Dark Matter-Baryon Scattering

We explore an interacting dark matter (IDM) model that allows for a fraction of dark matter (DM) to undergo velocity-independent scattering with baryons. In this scenario, structure on small scales is suppressed relative to the cold DM scenario. Using the effective field theory of large-scale structure, we perform the first systematic analysis of BOSS full-shape galaxy clustering data for the IDM scenario, and we find that this model alleviates the $S_8$ tension between large-scale structure and Planck data. Adding the $S_8$ prior from DES to our analysis further leads to a mild $\sim3\sigma$ preference for a non-vanishing DM-baryon scattering cross-section, assuming $\sim 10\%$ of DM is interacting and has a particle mass of 1 MeV. This result produces a modest $\sim 20$% suppression of the linear power at $k\lesssim 1~h$/Mpc, consistent with other small-scale structure observations. Similar scale-dependent power suppression was previously shown to have the potential to resolve $S_8$ tension between cosmological data sets. The validity of the specific IDM model explored here will be critically tested with upcoming galaxy surveys at the interaction level needed to alleviate the $S_8$ tension.