Searching for Dark Matter Interactions with ACT, SPT and DES

Models of a dark radiation sector with a mass threshold (WZDR+) have proved to be an appealing alternative to $\Lambda$CDM. These models provide simple comparison models, grounded in well-understood particle physics and with limited additional parameters. In addition, they have shown relevance in easing existing cosmological tensions, specifically the $H_0$ tension and the $S_8$ tension. Recently, measurements of CMB lensing by the ACT collaboration have provided strong additional information on clustering at late times. Within $\Lambda$CDM, these results yield a high value of $S_8$ at odds with weak-lensing measurements. In this work, we study this in the context of WZDR+, and find a much wider range of allowed values of $S_8$, and in particular much better agreement between data sets and an overall improvement of fit versus $\Lambda$CDM. We expand our analyses to include a wide set of data, including the ACT-DR6 lensing data, as well as primary CMB information from ACT-DR4 and SPT-3G, scale-dependent power spectra from DES and measurements of $H_0$ from SH0ES. We find that there is little to no tension in measurements of structure within the data sets, and the inferred value of $S_8$ is generally lower than that in $\Lambda$CDM. We find that the inclusion of DES generally favors a higher $H_0$, but there is some direct tension between the high-$\ell$ multipole data and this result. Future data should clarify whether this is a statistical artifact, or a true incompatibility of these datasets within this model.