Differentiating hidden sector dark matter from light WIMPs with
  Germanium detectors
  Light WIMP dark matter and hidden sector dark matter have been proposed to
explain the DAMA, CoGeNT and CRESST-II data. Both of these approaches feature
spin independent elastic scattering of dark matter particles on nuclei. Light
WIMP dark matter invokes a single particle species which interacts with
ordinary matter via contact interactions. By contrast hidden sector dark matter
is typically multi-component and is assumed to interact via the exchange of a
massless mediator. Such hidden sector dark matter thereby predicts a sharply
rising nuclear recoil spectrum, $dR/dE_R \sim 1/E_R^2$ due to this dynamics,
while WIMP dark matter predicts a spectrum which depends sensitively on the
WIMP mass, $m_\chi$. We compare and contrast these two very different possible
origins of the CoGeNT low energy excess. In the relevant energy range, the
recoil spectra predicted by these two theories approximately agree provided
$m_\chi \simeq 8.5$ GeV - close to the value favoured from fits to the CoGeNT
and CDMS low energy data. Forthcoming experiments including C-4, CDEX, and the
MAJORANA demonstrator, are expected to provide reasonably precise measurements
of the low energy Germanium recoil spectrum, including the annual modulation
amplitude, which should differentiate between these two theoretical
possibilities.
