Anisotropic diffusion of electrons in liquid xenon with application to improving the sensitivity of direct dark matter searches

TL;DR

This paper demonstrates that decreasing the electric field in liquid xenon detectors can significantly increase electron diffusion, improving background discrimination in dark matter searches, and provides the first measurement of the anisotropic diffusion coefficient.

Contribution

It reveals that lowering the electric field enhances electron diffusion, contrary to conventional practice, and provides the first measurement of the anisotropic diffusion coefficient in liquid xenon.

Findings

Decreasing electric field increases longitudinal electron diffusion.

Electron diffusion in liquid xenon is highly anisotropic.

First measurement of the longitudinal diffusion coefficient.

Abstract

Electron diffusion in a liquid xenon time projection chamber has recently been used to infer the $z$ coordinate of a particle interaction, from the width of the electron signal. The goal of this technique is to reduce the background event rate by discriminating edge events from bulk events. Analyses of dark matter search data which employ it would benefit from increased longitudinal electron diffusion. We show that a significant increase is expected if the applied electric field is decreased. This observation is trivial to implement but runs contrary to conventional wisdom and practice. We also extract a first measurement of the longitudinal diffusion coefficient, and confirm the expectation that electron diffusion in liquid xenon is highly anisotropic under typical operating conditions.