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 DM candidate, a stable weakly interacting massive particle (WIMP) of mass m(chi) = 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 . The strongest constraints are obtained from Monte Carlo Markov chain analysis of the combined Wilkinson Microwave Anisotropy Probe 7 and South Pole Telescope (SPT) data sets up to l(max) = 3100. If annihilation occurs via the e(+) - e(-) channel, a light WIMP can be excluded at the 2 sigma confidence level as a viable DM candidate in the above mass range. However, if annihilation occurs via mu(+) - mu(-) or tau(+) - tau(-) channel instead, we find that WIMPs with m(chi) > 5GeV might represent a viable cosmological DM candidate. We compare the results obtained in this work with those obtained adopting an analytical simplified model for the energy deposition process widely used in the literature, and we found that realistic energy deposition descriptions can influence the resulting constraints up to 60 per cent.