Exothermic Dark Matter
  We propose a novel mechanism for dark matter to explain the observed annual
modulation signal at DAMA/LIBRA which avoids existing constraints from every
other dark matter direct detection experiment including CRESST, CDMS, and
XENON10. The dark matter consists of at least two light states with mass ~few
GeV and splittings ~5 keV. It is natural for the heavier states to be
cosmologically long-lived and to make up an O(1) fraction of the dark matter.
Direct detection rates are dominated by the exothermic reactions in which an
excited dark matter state down-scatters off of a nucleus, becoming a lower
energy state. In contrast to (endothermic) inelastic dark matter, the most
sensitive experiments for exothermic dark matter are those with light nuclei
and low threshold energies. Interestingly, this model can also naturally
account for the observed low-energy events at CoGeNT. The only significant
constraint on the model arises from the DAMA/LIBRA unmodulated spectrum but it
can be tested in the near future by a low-threshold analysis of CDMS-Si and
possibly other experiments including CRESST, COUPP, and XENON100.
