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Cosmic microwave background constraints on light dark matter candidates

Unveiling the nature of cosmic dark matter (DM) is an urgent issue in cosmology. Here we make use of a strategy based on the search for the imprints left on the cosmic microwave background temperature and polarization spectra by the energy deposition due to annihilations of the most promising dark matter candidate, a stable WIMP of mass 1-20 GeV. A major improvement with respect to previous similar studies is a detailed treatment of the annihilation cascade and its energy deposition in the cosmic gas. This is vital as this quantity is degenerate with the annihilation cross-section <σv>. The strongest constraints are obtained from Monte Carlo Markov chain analysis of the combined WMAP7 and SPT datasets up to lmax = 3100. If annihilation occurs via the e+e- channel, a light WIMP can be excluded at the 2 σ c.l. as a viable DM candidate in the above mass range. However, if annihilation occurs via μ+μ- or τ+τ- channels instead we find that WIMPs with mass > 5 GeV might represent a viable cosmological DM candidate. We compare the results obtained in the present work with those obtained adopting an analytical simplified model for the energy deposition process widely used in literature, and we found that realistic energy deposition descriptions can influence the resulting constrains up to 60%.