Single Photoelectron Detection Efficiency in the LUX-ZEPLIN Experiment's PMT Systems*

The LUX-ZEPLIN (LZ) experiment is a dark matter detector deployed at the Sanford Underground Research Facility in Lead, South Dakota. LZ is instrumented with 745 photomultiplier tubes (PMTs) in three sub-detectors. To be sensitive to low-energy nuclear recoil (NR) events, LZ requires a high single photoelectron (SPHE) detection efficiency in the liquid xenon time projection chamber (TPC). We characterize the SPHE detection efficiency using an LED calibration system. These calibrations are vital to monitoring the health of PMTs and the LZ signal chain.

During commissioning, the high SPHE detection efficiency of the as-installed LZ experiment allowed us to explore reducing PMT gain. These studies indicated that the low-noise environment of the signal chain permitted a reduction of PMT gain while maintaining sensitivity to our science objectives. It was found that reducing PMT gain allowed us to resolve pathological behavior exhibited by some PMTs.

In this work, we motivate our study of SPHE detection efficiency by quantifying the impact on low-energy NR events. We present an overview of how LZ's SPHE detection efficiency is measured. Finally, we show the time evolution of SPHE detection efficiency in each of LZ's sub-detectors.