Title: A new step in interacting dark sector cosmologies

Abstract: With the growing precision of cosmological measurements, tensions in the determination of cosmological parameters have arisen that might be the first manifestations of physics going beyond Lambda CDM. We propose a new class of interacting dark sector models, which lead to qualitatively new cosmological behavior, which can potentially simultaneously address the two most important tensions in cosmological data, the H0 and S8 tensions. The main ingredient in this class of models is multi-component interacting dark radiation, with at least one of the components having a light mass and transitioning to non-relativistic close to the time of matter-radiation equality, and interactions between this massive dark radiation component and a fraction of dark matter. This generalizes the scenario known as Partially Acoustic Dark Matter, in which a fraction of dark matter has strong interactions with dark radiation throughout most of the universe evolution. By introducing a mass-threshold for dark radiation, in which the component that interacts with dark matter becomes non-relativistic and quickly annihilates into the massless component, we achieve two goals, each of them helping improve one of the aforementioned tensions and leaving the power spectrum identical to Lambda CDM at larger scales.