Electron extraction efficiency study for dual-phase xenon dark matter experiments

Dual-phase xenon detectors are widely used in dark matter direct detection experiments, and have demonstrated the highest sensitivities to a variety of dark matter interactions. However, a key component of the dual-phase detector technology--the efficiency of charge extraction from liquid xenon into gas--has not been well characterized. In this paper, we report a new measurement of the electron extraction efficiency (EEE) in a small xenon detector using two mono-energetic decay features of $^{37}$Ar. By achieving stable operation at very high voltages, we measured the EEE values at the highest extraction electric field strength reported to date. For the first time, an apparent saturation of the EEE is observed over a large range of electric field; between 7.5 kV/cm and 10.4 kV/cm extraction field in the liquid xenon the EEE stays stable at the level of 1%(kV/cm)$^{-1}$. In the context of electron transport models developed for xenon, we discuss how the observed saturation may help calibrate this relative EEE measurement to the absolute EEE values. In addition, we present the implications of this result not only to current and future xenon-based dark matter searches, but also to xenon-based searches for coherent elastic neutrino-nucleus scatters.