Full-shape BOSS constraints on dark matter interacting with dark radiation and lifting the $S_8$ tension

In this work we derive constraints on interacting dark matter-dark radiation models from a full-shape analysis of BOSS-DR12 galaxy clustering data, combined with Planck legacy cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) measurements. We consider a set of models parameterized within the effective theory of structure formation (ETHOS), quantifying the lifting of the $S_8$ tension in view of KiDS weak-lensing results. The most favorable scenarios point to a fraction $f\sim 10-100\%$ of interacting dark matter as well as a dark radiation temperature that is smaller by a factor $ξ\sim 0.1-0.15$ compared to the CMB, leading to a reduction of the tension to the $\sim 1σ$ level. The temperature dependence of the interaction rate favored by relaxing the $S_8$ tension is realized for a weakly coupled unbroken non-Abelian $SU(N)$ gauge interaction in the dark sector. To map our results onto this $SU(N)$ model, we compute higher-order corrections due to Debye screening. We find a lower bound $α_d\equiv g_d^2/(4π)\gtrsim 10^{-8} (10^{-9})$ for dark matter mass $1000 (1)$ GeV for relaxing the $S_8$ tension, consistent with upper bounds from galaxy ellipticities and compatible with self-interactions relevant for small-scale structure formation.