  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 <{\sigma}v>. 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 {\sigma} c.l. as a viable DM
candidate in the above mass range. However, if annihilation occurs via
{\mu}+{\mu}- or {\tau}+{\tau}- 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%.
