Measurement of the ionization yield from nuclear recoils in liquid xenon between 0.3 -- 6 keV with single-ionization-electron sensitivity

TL;DR

This study precisely measures the ionization yield from nuclear recoils in liquid xenon at energies as low as 0.3 keV, providing critical calibration data for dark matter and neutrino detection experiments.

Contribution

It presents the lowest energy calibration of ionization yield in liquid xenon and explores its dependence on electric drift fields, improving the understanding of detector responses at very low energies.

Findings

Ionization yield measured down to 0.3 keV, the lowest reported to date.

Agreement with existing data between 2 and 6 keV, with improved precision.

Decreasing ionization yield trend observed below 2 keV, deviating from simple extrapolations.

Abstract

Dual-phase xenon TPC detectors are a highly scalable and widely used technology to search for low-energy nuclear recoil signals from WIMP dark matter or coherent nuclear scattering of $\sim$MeV neutrinos. Such experiments expect to measure O(keV) ionization or scintillation signals from such sources. However, at $\sim1\,$keV and below, the signal calibrations in liquid xenon carry large uncertainties that directly impact the assumed sensitivity of existing and future experiments. In this work, we report a new measurement of the ionization yield of nuclear recoil signals in liquid xenon down to 0.3$\,$keV$\,\,$-- the lowest energy calibration reported to date -- at which energy the average event produces just 1.1~ionized~electrons. Between 2 and 6$\,$keV, our measurements agree with existing measurements, but significantly improve the precision. At lower energies, we observe a decreasing trend that deviates from simple extrapolations of existing data. We also study the dependence of ionization yield on the applied drift field in liquid xenon between 220V/cm and 6240V/cm, allowing these measurements to apply to a broad range of current and proposed experiments with different operating parameters.