  We present a novel analysis technique for liquid xenon time projection
chambers that allows for a lower threshold by relying on events with a prompt
scintillation signal consisting of single detected photons. The energy
threshold of the LUX dark matter experiment is primarily determined by the
smallest scintillation response detectable, which previously required a 2-fold
coincidence signal in its photomultiplier arrays, enforced in data analysis.
The technique presented here exploits the double photoelectron emission effect
observed in some photomultiplier models at vacuum ultraviolet wavelengths. We
demonstrate this analysis using an electron recoil calibration dataset and
place new constraints on the spin-independent scattering cross section of
weakly interacting massive particles (WIMPs) down to 2.5 GeV/c$^2$ WIMP mass
using the 2013 LUX dataset. This new technique is promising to enhance light
WIMP and astrophysical neutrino searches in next-generation liquid xenon
experiments.
