Revisiting CMB constraints on dark matter annihilation
  The precision measurements of the cosmic microwave background power spectrum
put a strong constraint on the dark matter annihilation cross section since the
electromagnetic energy injection by the dark matter annihilation affects the
ionization history of the universe. In this paper, we update our previous
simulation code for calculating the ionization history with the effect of dark
matter annihilation by including Helium interactions and improving the
precision of calculations. We give an updated constraint on the annihilation
cross section and mass of dark matter using the modified RECFAST code with the
Planck 2018 datasets.
