  The LUX dark matter search experiment is a 350 kg dual-phase xenon time
projection chamber located at the 4850 ft level of the Sanford Underground
Research Facility in Lead, SD. The success of two-phase xenon detectors for
dark matter searches relies on their ability to distinguish electron recoil
(ER) background events from nuclear recoil (NR) signal events. Typically, the
NR-ER discrimination is obtained from the ratio of the electroluminescence
light (S2) to the prompt scintillation light (S1). Analysis of the S1 pulse
shape is an additional discrimination technique that can be used to distinguish
NR from ER. Pulse-shape NR-ER discrimination can be achieved based on the ratio
of the de-excitation processes from singlet and triplet states that generate
the S1. The NR S1 is dominated by the de-excitation process from singlet states
with a time constant of about 3 ns while the ER S1 is dominated by the
de-excitation process from triplet states with a time constant of about 24 ns.
As the size of the detectors increases, the variation in the S1 photon flight
path can become comparable to these decay constants, reducing the utility of
pulse-shape analysis to separate NR from ER. The effect of path length
variations in the LUX detector has been studied using the results of
simulations and the impact on the S1 pulse shape analysis is discussed.

Instrumentation and Detectors;High Energy Physics - Experiment