Dual-phase noble gas Time Projection Chambers (TPCs) suffer from spurious electron background events at the lowest detectable energy region. This background is reported in liquid xenon TPCs and some of the causes are discussed in the literature. Understanding its origin is of paramount importance as this background sets the analysis threshold and affects the most sensitive part of the region of interest for low mass dark matter searches.
We report the spurious electron background events observed in the liquid argon TPC in the DarkSide-50 experiment. We found two different electron populations based on time correlation with preceding events: simultaneous emission and delayed emission. The majority of the former can be associated with photoionization effects. The mechanism of the latter is not clear, but our observations indicate that they are related to the impurity level in the TPC measured via the electron lifetime.