High single photoelectron (SPE) rates have been recently observed in several large LArTPC detectors operating at the surface. These single photons are background that could limit the LArTPC capability for low energy physics. To understand the origin of the high SPE rate, we developed a model focusing on the slowly moving positive and negative ions (space charge) induced by the ionization of cosmic rays and electron attachment to impurities in LAr. Our model can predict the electric field distortion due to the presence of space charge that is well matched to measurements. More importantly, the model contains two new processes that contribute to producing photons from the interactions of the ions, which provide a plausible explanation for the observed high single PE rate.
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