Response of liquid xenon to Compton electrons down to 1.5 keV

TL;DR

This study investigates liquid xenon's scintillation response to low-energy electronic recoils down to 1.5 keV, providing insights crucial for dark matter detection sensitivity and threshold estimation.

Contribution

It presents new measurements of liquid xenon scintillation at very low energies with and without electric fields, extending previous results and refining detector thresholds.

Findings

Scintillation yield drops to 40% at zero field below 10 keV

Electric field reduces scintillation to about 75% of zero-field value

Threshold estimates for various detectors confirm high sensitivity to low-energy recoils

Abstract

The response of liquid xenon to low-energy electronic recoils is relevant in the search for dark-matter candidates which interact predominantly with atomic electrons in the medium, such as axions or axion-like particles, as opposed to weakly interacting massive particles which are predicted to scatter with atomic nuclei. Recently, liquid-xenon scintillation light has been observed from electronic recoils down to 2.1 keV, but without applied electric fields that are used in most xenon dark matter searches. Applied electric fields can reduce the scintillation yield by hindering the electron-ion recombination process that produces most of the scintillation photons. We present new results of liquid xenon's scintillation emission in response to electronic recoils as low as 1.5 keV, with and without an applied electric field. At zero field, a reduced scintillation output per unit deposited energy is observed below 10 keV, dropping to nearly 40% of its value at higher energies. With an applied electric field of 450 V/cm, we observe a reduction of the scintillation output to about 75% relative to the value at zero field. We see no significant energy dependence of this value between 1.5 keV and 7.8 keV. With these results, we estimate the electronic-recoil energy thresholds of ZEPLIN-III, XENON10, XENON100, and XMASS to be 2.8 keV, 2.5 keV, 2.3 keV, and 1.1 keV, respectively, validating their excellent sensitivity to low-energy electronic recoils.