  The determination of dark matter constraints from liquid xenon direct
detection experiments depends upon the amount of scintillation light produced
by nuclear recoils in the detector, a quantity that is characterized by the
scintillation efficiency factor Leff. We examine how uncertainties in the
measurements of Leff and the extrapolated behavior of Leff at low recoil
energies (where measurements do not exist) affect the constraints from
experiments such as XENON10 and XENON100, particularly in the light WIMP
regions of interest for the DAMA and CoGeNT experimental results.
