Light scalar WIMP through the Higgs portal and CoGeNT

If dark matter (DM) simply consists in a scalar particle interacting dominantly with the Higgs boson, the ratio of its annihilation cross section-which is relevant both for the relic abundance and indirect detection-and its spin-independent scattering cross section on nuclei depends only on the DM mass. It is an intriguing result that, fixing the mass and direct detection rate to fit the annual modulation observed by the DAMA experiment, one obtains a relic density in perfect agreement with its observed value. In this article we update this result and confront the model to the recent CoGeNT data, tentatively interpreting the excess of events in the recoil energy spectrum as being due to DM. CoGeNT, as DAMA, points toward a light DM candidate, with somewhat different (but not necessarily incompatible) masses and cross sections. For the CoGeNT region too, we find an intriguing agreement between the scalar DM relic density and direct detection constraints. We give the 1 sigma region favored by the CDMS-II events, and our exclusion limits for the Xenon10 (2009) and Xenon100 data, which, depending on the scintillation efficiency, may exclude CoGeNT and DAMA. Assuming CoGeNT and/or DAMA to be due to scalar singlet DM leads to definite predictions regarding indirect detection and at colliders. We specifically emphasize the limit on the model that might be set by the current Fermi-LAT data on dwarf galaxies, and the implications for the search for the Higgs at the LHC.