Precision measurements of the scintillation pulse shape for low-energy recoils in liquid xenon

TL;DR

This study measures the scintillation pulse shapes in liquid xenon for low-energy nuclear and electronic recoils, analyzing their decay components and discrimination potential for dark matter detection.

Contribution

It provides detailed pulse shape measurements and models for low-energy recoils in liquid xenon, assessing their usefulness for background discrimination.

Findings

Pulse shape broadening for electronic recoils due to recombination.

Effective two-exponential decay model describes pulse shapes well.

Pulse shape discrimination offers limited improvement over S2/S1 methods.

Abstract

We present measurements of the scintillation pulse shape in liquid xenon for nuclear recoils (NR) and electronic recoils (ER) at electric fields of 0 to 0.5 kV/cm for energies $<$ 15 keV and $<$ 70 keV electron-equivalent, respectively. The average pulse shapes are well-described by an effective model with two exponential decay components, where both decay times are fit parameters. We find significant broadening of the pulse for ER due to delayed luminescence from the recombination process. In addition to the effective model, we fit a model describing the recombination luminescence for ER at zero field and obtain good agreement. We estimate the best performance of a combined S2/S1 and pulse shape ER/NR discrimination and show that even with 2 ns time resolution, the improvement over S2/S1 discrimination alone is marginal, so that pulse shape discrimination will likely not be useful for future dual-phase liquid xenon experiments looking for elastic dark matter recoil interactions.