  Liquid Xenon (LXe) is an excellent material for experiments designed to
detect dark matter in the form of Weakly Interacting Massive Particles (WIMPs).
A low energy detection threshold is essential for a sensitive WIMP search. The
understanding of the relative scintillation efficiency (Leff) and ionization
yield of low energy nuclear recoils in LXe is limited for energies below 10
keV. In this paper, we present new measurements that extend the energy down to
4 keV, finding that Leff decreases with decreasing energy. We also measure the
quenching of scintillation efficiency due to the electric field in LXe, finding
no significant field dependence.
