First measurement of surface nuclear recoil background for argon dark matter searches

TL;DR

This paper characterizes the surface nuclear recoil background from radon progeny in liquid argon dark matter detectors, demonstrating significant suppression techniques to improve detection sensitivity for WIMPs.

Contribution

First measurement and analysis of surface $^{206}$Pb recoil background in liquid argon, showing effective suppression methods for future dark matter searches.

Findings

Pb recoil signals are highly quenched in argon.

Surface background can be suppressed by ~100 times using pulse shape discrimination.

This suppression enables near background-free dark matter detection.

Abstract

One major background in direct searches for weakly interacting massive particles (WIMPs) comes from the deposition of radon progeny on detector surfaces. The most dangerous surface background is the $^{206}$Pb recoils produced by $^{210}$Po decays. In this letter, we report the first characterization of this background in liquid argon. The scintillation signal of low energy Pb recoils is measured to be highly quenched in argon, and we estimate that the 103keV $^{206}$Pb recoil background will produce a signal equal to that of a ~5keV (30keV) electron recoil ($^{40}$Ar recoil). In addition, we demonstrate that this dangerous $^{210}$Po surface background can be suppressed by a factor of ~100 or higher using pulse shape discrimination methods, which can make argon dark matter detectors near background-free and enhance their potential for discovery of medium- and high-mass WIMPs. We also discuss the impact on other low background experiments.