  The presence of light thermally coupled dark matter affects early expansion
history and production of light elements during the Big Bang Nucleosynthesis.
Specifically, dark matter that annihilates into Standard Model particles can
modify the effective number of light species in the universe $N_\mathrm{eff}$,
as well as the abundance of light elements created buring BBN. These quantities
in turn affect the cosmic microwave background (CMB) anisotropy. We present the
first joint analysis of small-scale temperature and polarization CMB anisotropy
from Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT), together
with Planck data and the recent primordial abundance measurements of helium and
deuterium to place comprehensive bounds on the mass of light thermal-relic dark
matter. We consider a range of models, including dark matter that couples to
photons and Standard-Model neutrinos. We find that the combination of ACT, SPT,
and Planck generally leads to the most stringent mass constraint for dark
matter that couples to neutrinos, improving the lower limit by 40%-80%, with
respect to previous Planck analyses. On the other hand, the addition of ACT and
SPT 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 with
primordial abundance measurements, we rule out masses below $\sim$4 MeV at 95%
confidence, for all models. We show that allowing for new relativistic species
can weaken the mass bounds for dark matter that couples to photons by up to an
order of magnitude or more. Finally, we discuss the reach of the next
generation of the CMB experiments in terms of probing the mass of the thermal
relic dark matter.
