New Measurement of the Scintillation Efficiency of Low-Energy Nuclear Recoils in Liquid Xenon

TL;DR

This work targets the dominant systematic in low-mass WIMP searches: the low-energy nuclear recoil scintillation efficiency in liquid xenon, $\mathcal{L}_{\mathrm{eff}}$. Using a high-light-yield LXe detector and monoenergetic neutrons with fixed-angle scattering, the authors directly extract $\mathcal{L}_{\mathrm{eff}}(E_{\mathrm{nr}})$ down to $3$ keV by fitting detailed Geant4-based simulations to the measured recoil spectra, corrected for trigger efficiency and detector response. They find $\mathcal{L}_{\mathrm{eff}}$ decreases gradually from $0.144 \pm 0.009$ at 15 keV to $0.088^{+0.014}_{-0.015}$ at 3 keV, with the main uncertainties arising from the recoil-energy spread and the trigger efficiency near threshold. The results provide an improved low-energy calibration for LXe dark matter detectors (e.g., XENON100) and suggest future work to jointly measure scintillation and ionization yields for enhanced recoil-energy reconstruction.

Abstract

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.

Figures (13)

• Figure 1: Schematic of the experimental setup. A sealed-tube neutron generator, where deuterons of energy $E_d$ are incident upon a titanium deuteride target, produces neutrons at various angles $\varphi$. Some of the neutrons emitted at an angle $\varphi = \frac{\pi}{2}$ scatter in the LXe detector at an angle $\theta$ and are tagged by two EJ301 organic liquid scintillators neutron detectors.
• Figure 2: Explosion drawing of the LXe detector.
• Figure 3: Xenon gas system used for continuous purification of the LXe.
• Figure 4: Measurement (points) and theoretical calculation (solid line) of the neutron generator flux as a function of high voltage (top) and beam current (down).
• Figure 5: Measured (points) and simulated (solid curve) efficiency of the 2-fold coincidence LXe trigger. The measured trigger efficiency is used to extract $\mathcal{L}_{\mathrm{eff}}$ (Sec. \ref{['sec:leff']}) from the neutron scattering measurements.