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The presence of light thermally coupled dark matter affects early expansionhistory and production of light elements during the Big Bang Nucleosynthesis.Specifically, dark matter that annihilates into Standard Model particles canmodify the effective number of light species in the universe $N_\mathrm{eff}$,as well as the abundance of light elements created buring BBN. These quantitiesin turn affect the cosmic microwave background (CMB) anisotropy. We present thefirst joint analysis of small-scale temperature and polarization CMB anisotropyfrom Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT), togetherwith Planck data and the recent primordial abundance measurements of helium anddeuterium to place comprehensive bounds on the mass of light thermal-relic darkmatter. We consider a range of models, including dark matter that couples tophotons and Standard-Model neutrinos. We find that the combination of ACT, SPT,and Planck generally leads to the most stringent mass constraint for darkmatter that couples to neutrinos, improving the lower limit by 40%-80%, withrespect to previous Planck analyses. On the other hand, the addition of ACT andSPT leads to a slightly weaker bound on electromagnetically coupled particles,due to a shift in the preferred values of $Y_\mathrm{p}$ and $N_\mathrm{eff}$driven by the ground based experiments. Combining all CMB measurements withprimordial abundance measurements, we rule out masses below $\sim$4 MeV at 95%confidence, for all models. We show that allowing for new relativistic speciescan weaken the mass bounds for dark matter that couples to photons by up to anorder of magnitude or more. Finally, we discuss the reach of the nextgeneration of the CMB experiments in terms of probing the mass of the thermalrelic dark matter.

What does cosmology tell us about the mass of thermal-relic dark matter?