  Particle detectors that use liquid xenon (LXe) as detection medium are among
the leading technologies in the search for dark matter weakly interacting
massive particles (WIMPs). A key enabling element has been the low-energy
detection threshold for recoiling nuclei produced by the interaction of WIMPs
in LXe targets. In these detectors, the nuclear recoil energy scale is based on
the LXe scintillation signal and thus requires knowledge of the relative
scintillation efficiency of nuclear recoils, Leff. The uncertainty in Leff at
low energies is the largest systematic uncertainty in the reported results from
LXe WIMP searches at low masses. In the context of the XENON Dark Matter
project, a new LXe scintillation detector has been designed and built
specifically for the measurement of Leff at low energies, with an emphasis on
maximizing the scintillation light detection efficiency to obtain the lowest
possible energy threshold. We report new measurements of Leff at low energies
performed with this detector. Our results suggest a Leff which slowly decreases
with energy, from 0.144 +/- 0.009 at 15 keV down to 0.088 +0.014 -0.015 at 3
keV.
