Pushing the Limits of Pulse Shape Discrimination in a Large Liquid Xenon Detector

The LUX-ZEPLIN (LZ) experiment is a direct-detection dark matter experiment, optimized to search for weakly interacting massive particles (WIMPs) through WIMP-nucleon interactions. The main challenge in dark matter detection is differentiating between WIMP signals and background events. In LZ, the ratio of ionization to scintillation signals (charge-to-light) is the primary method for rejecting electronic recoil (ER) background. Pulse shape discrimination (PSD) offers a method for additional ER backgrounds rejection in liquid xenon detectors. In this paper, the discrimination power of PSD with the LZ experiment is discussed. To precisely characterize the scintillation pulse shape, an analysis framework is developed to reconstruct the detection time of individual photons. Using LZ calibration data, the photon-timing prompt fraction discriminator is optimized and achieves ER leakage as low as $15%$. For specific background processes such as $^{124}$Xe double electron capture, the leakage is reduced further to about $5%$. PSD is combined with charge-to-light to form two-factor discrimination (TFD). The optimized TFD performance is compared with the performance of the charge-to-light method, with the corresponding false positive rate reduced by up to a factor of two for large scintillation pulses. Finally, PSD and TFD are applied to data from LZ’s WS2024 run and their performance is summarized.

💡 Research Summary

The LUX‑ZEPLIN (LZ) experiment searches for weakly interacting massive particles (WIMPs) using a 7‑ton liquid xenon (LXe) time‑projection chamber. While the traditional charge‑to‑light ratio provides strong discrimination between nuclear recoils (NR) and electronic recoils (ER), additional background rejection is desirable to improve signal acceptance. This paper demonstrates that pulse‑shape discrimination (PSD) can be effectively employed in LXe by reconstructing the arrival times of individual scintillation photons with sub‑nanosecond precision.

First, channel‑by‑channel timing offsets are calibrated using an LED system. By measuring the 20 % rise‑time of each PMT’s single‑photoelectron template and correcting for photon time‑of‑flight, relative timing accuracies better than 1 ns are achieved. A novel “N‑photon model” then fits each PMT waveform with up to three single‑photoelectron templates, extracting photon amplitudes and arrival times via a Bayesian selection of the most probable photon count.

Using these reconstructed photon‑time spectra, a prompt‑fraction (PF) discriminator is defined as the fraction of photons arriving within the first ~30 ns of the S1 pulse. Calibration data (tritiated methane, DD neutrons) show that PF separates ER from NR, achieving an overall ER leakage of ~15 % while reducing leakage to ~5 % for specific backgrounds such as 124Xe double‑electron capture.

The PF is combined with the conventional charge‑to‑light ratio to form a two‑factor discriminator (TFD). Optimizing the two‑dimensional cut yields a receiver‑operating‑characteristic (ROC) curve where, for large S1 signals (>100 phd), the false‑positive rate is reduced by up to a factor of two relative to charge‑to‑light alone, without sacrificing NR efficiency.

Finally, the TFD is applied to the WS2024 dataset (March 2023–April 2024). The observed ER rejection matches the calibration expectations, confirming that PSD provides a robust, independent background‑rejection dimension in a large LXe detector. The paper also integrates the photon‑timing model into the NEST simulation framework, enabling realistic predictions for future detector designs. Overall, precise photon‑timing reconstruction and combined PSD/charge‑to‑light discrimination significantly extend the background‑rejection capabilities of LXe dark‑matter experiments.