Distinguishing standard reionization from dark matter models

The Wilkinson Microwave Anisotropy Probe (WMAP) experiment has detected reionization at the 5.5 sigma level and has reported a mean optical depth of 0.088 +/- 0.015. A powerful probe of reionization is the large-angle EE polarization power spectrum, which is now ( since the first five years of data from WMAP) cosmic variance limited for 2 <= l <= 6. Here we consider partial reionization caused by weakly interacting massive particle dark matter annihilation, and calculate the expected polarization power spectrum. We compare the dark matter models with a standard two-step reionization theory, and examine whether the models may be distinguished using current, and future cosmic microwave background (CMB) observations. We consider dark matter annihilation at intermediate redshifts (z < 60) due to halos, as well as annihilation at higher redshifts due to free particles. In order to study the effect of high redshift dark matter annihilation on CMB power spectra, it is essential to include the contribution of residual electrons ( left over from recombination) to the ionization history. Dark matter halos at redshifts z < 60 influence the low multipoles l < 20 in the EE power spectrum, while the annihilation of free particle dark matter at high redshifts z > 100 mainly affects multipoles l > 10.