Electron train backgrounds in liquid xenon dark matter search detectors are indeed due to thermalization and trapping

Abstract: Electron emission from liquid into gaseous xenon is a cornerstone of dark matter search detectors such as ZEPLIN, XENON, LUX and LZ. The probability of emission is a function of the applied electric field E, and electrons which fail to pass from the liquid into the gas have been previously hypothesized to become thermalized and trapped. This article shows, for the first time, quantitative agreement between an electron emission model and existing data. The model predicts that electrons in the liquid must surmount a typical potential barrier phi_b=0.34+-0.01 eV in order to escape into the gas. This value is a factor of about x2 smaller than has previously been calculated or inferred. Knowledge of phi_b allows calculation of the lifetime of thermalized, trapped electrons. The value is O(10) ms, which appears to be compatible with XENON10 observations of electron train backgrounds. As these backgrounds limit the sensitivity of dark sector dark matter searches, possible mitigations are discussed.