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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 LZ experiment 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.
