Constraints on Dark Matter-Dark Energy Scattering from ACT DR6 CMB Lensing

The predicted present-day amplitude of matter fluctuations based on cosmic microwave background (CMB) anisotropy data has sometimes been found discrepant with more direct measurements of late-time structure. This has motivated many extensions to the standard cosmological model, including kinetic interactions between dark matter and dark energy that introduce a drag force slowing the growth of structure at late times. Exploring this scenario, we develop a model for quasi-linear scales in the matter power spectrum by calculating the critical overdensity in the presence of this interaction and a varying dark energy equation of state. We explicitly avoid modeling or interpretation of data on non-linear scales in this model (such as use of ΛCDM-calibrated priors), which would require numerical simulations. We find that the presence of the drag force hinders halo formation, thus increasing the deviation from ΛCDM in the mildly non-linear regime. We use CMB lensing observations from the sixth data release of the Atacama Cosmology Telescope up to L=1250 (in combination with Planck, Sloan Digital Sky Survey, and 6dFGS data) to derive the strongest constraints to date on the amplitude of the drag term, finding the dimensionless interaction strength Γ_DMDE/(H_0ρ_c)<0.831 (2.81) at the 68% (95%) confidence level. The inclusion of non-linear corrections improves our constraints by about 35% compared to linear theory. Our results do not exclude the best-fit values of Γ_DMDE found in previous studies using information from galaxy weak lensing, though we find no statistical preference for the dark matter-dark energy kinetic interactions over ΛCDM. We implement our model in a publicly available fork of the Boltzmann code CLASS at https://github.com/fmccarthy/Class_DMDE.