Towards an improved understanding of the relative scintillation efficiency of nuclear recoils in liquid xenon

Abstract

Liquid xenon (LXe) particle detectors are a powerful technology in the field of dark matter direct detection, having shown impressive results in recent years and holding strong possibility for leading the field in sensitivity to galactic weakly interacting massive particles (WIMPs) in the future. The search for WIMPs requires the capability to detect the recoiling nuclei that result when these particles interact with normal matter. In order to make meaningful statements about an observed signal, or lack thereof, the energy scale of recoiling nuclei in LXe must be known. Our understanding of this energy scale is contained in a quantity called the relative scintillation efficiency of nuclear recoils, or L_eff, and has been studied extensively in the literature, producing seemingly contradictory results. I examine all the measurements of L_eff that exist, both direct and indirect, and extract the energy dependent behavior that is statistically consistent globally with all values. Additionally, I examine the measurements covering low energies (>~10 keV, where the largest disagreements exist) and attempt to diagnose the systematic effects that have led to the observed inconsistencies. I show that virtually all major disparity arises due to efficiency roll-off of the detectors at the low energies, and, when taking this into account, find that the observed behavior of L_eff supports a slowly and smoothly decreasing value with decreasing energy. Finally, I discuss the prospects for future measurements, and derive a practical limit to what can be achieved.