Updated CMB constraints on dark matter annihilation cross sections

The injection of secondary particles produced by dark matter (DM) annihilation at redshift 100 less than or similar to z less than or similar to 1000 affects the process of recombination, leaving an imprint on cosmic microwave background (CMB) anisotropies. Here we provide a new assessment of the constraints set by cosmic microwave background data on the mass and self-annihilation cross section of DM particles. Our new analysis includes the most recent WMAP (7-year) and ACT data, as well as an improved treatment of the time-dependent coupling between the DM annihilation energy with the thermal gas. We show, in particular, that the improved measurement of the polarization signal places already stringent constraints on light DM particles, ruling out thermal WIMPs with mass m(chi) less than or similar to 10 GeV annihilating into electrons and WIMPs with mass m(chi) less than or similar to 4 GeV annihilating into muons.