  Scintillation efficiency of low-energy nuclear recoils in noble liquids plays
a crucial role in interpreting results from some direct searches for Weakly
Interacting Massive Particle (WIMP) dark matter. However, the cause of a
reduced scintillation efficiency relative to electronic recoils in noble
liquids remains unclear at the moment. We attribute such a reduction of
scintillation efficiency to two major mechanisms: 1) energy loss and 2)
scintillation quenching. The former is commonly described by Lindhard's theory
and the latter by Birk's saturation law. We propose to combine these two to
explain the observed reduction of scintillation yield for nuclear recoils in
noble liquids. Birk's constants $kB$ for argon, neon and xenon determined from
existing data are used to predict noble liquid scintillator's response to
low-energy nuclear recoils and low-energy electrons. We find that energy loss
due to nuclear stopping power that contributes little to ionization and
excitation is the dominant reduction mechanism in scintillation efficiency for
nuclear recoils, but that significant additional quenching results from the
nonlinear response of scintillation to the ionization density.
