  The discrepancies in claims of experimental evidence in the search for weakly
interacting massive particle (WIMP) dark matter necessitate a model for
ionization efficiency (the quenching factor) at energies below 10 keV. We have
carefully studied the physics processes that contribute to the ionization
efficiency through stopping power. The focus of this work is the construction
of a model for the ionization efficiency in germanium by analyzing the
components of stopping power, specifically that of the nuclear stopping power,
at low energies. We find a fraction of the ZBL nuclear stopping power can
contribute to ionization efficiency. We propose a model that corrects the
missing contribution to ionization efficiency from the ZBL nuclear stopping
power. The proposed model is compared to previous measurements of ionization
efficiency in germanium as well as that of other theoretical models. Using this
new model, the thresholds of both CDMS II and CoGeNT are analyzed and compared
in terms of the nuclear recoil energy.
