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The LZ detector learns to recognize dark matter particles by the shape of their light flash.
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Underground, in a former gold mine, sits a tank of liquid xenon. If a dark matter particle flies through it, the xenon flashes. But it also flashes from ordinary particles, like neutrinos or leftover radioactivity. The task is to tell which flash comes from dark matter and which from noise.
Scientists on the LZexperiment found a way: the flash has a shape. Dark matter particles are expected to strike an atom's nucleus, and the light fades more slowly. Ordinary electrons give a quicker flash. The detector measures the arrival time of each photon, like a stopwatch for every ray of light. This shape can filter out some background events.
Previously, scientists looked only at the ratio of light to charge. Now they've added the flash shape — a double filter.
The method was tested on calibration data. For a rare background — double electron capture in xenon-124—the background leakage dropped to 5%. And when combining the flash shape with the previous method, false triggers become half as frequent for bright flashes.
So the LZ detector is becoming more attentive: it doesn't just see how much light there is, but also how that light changes over time. Maybe this is exactly how the first dark matter particle will be caught.
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.
