The impacts of dark matter particle annihilation on recombination and   the anisotropies of the cosmic microwave background
  The recombination history of the Universe provides a useful tool for constraining the annihilation of dark matter particles. Even a small fraction of dark matter particles annihilated during the cosmic dark age can provide sufficient energy to affect the ionization state of the baryonic gas. Although this effect is too small for neutralinos, lighter dark matter particle candidates, e.g. with mass of 1-100 MeV, which was proposed recently to explain the observed excess of positrons in the Galactic Center, may generate observable differences in the cosmic microwave background (CMB) temperature and polarization anisotropies. The annihilations at the era of recombination affects mainly the CMB anisotropy at small angular scales (large $\ell$), and is distinctively different from the effect of early reionization. We perform a multi-parameter analysis of the CMB data, including the both the WMAP first year and three year data, and the ACBAR, Boomerang, CBI, and VSA data. Assuming that the observed excess of $e^+e^-$ pairs in the galactic center region is produced by dark matter annihilation, and that a sizable fraction of the energy produced in the annihilation is deposited in the baryonic gas during recombination, we obtain a %95 dark matter mass limit of $M<8 \MeV$ with the current data set. 
Le Zhang, Xuelei Chen, Yi-An Lei, Zongguo Si