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A liquid argon detector searched for dark matter particles but only found dust specks.
Deep in a Canadian mine, there's a barrel holding 3269 kilograms of liquid argon — a clear fluid that looks like very cold water. This barrel is the DEAP-3600 detector, and its job is to catch dark matter particles, which are thought to permeate the universe but barely interact with ordinary matter.
Over 790.8 days of observation, the detector recorded no convincing dark matter signal. But it did pick up plenty of false signals from tiny dust specks floating in the argon. These specks emit alpha particles that the detector mistakes for the sought-after particles. To tell them apart, scientists used three clues: the event's energy, the shape of the light pulse, and where it occurred.
Dark matter doesn't emit or reflect light: it can only be noticed through gravity. That's why detectors are buried deep underground to block cosmic rays.
The new analysis ruled out some mass ranges for dark matter particles. For example, for a mass of 100 GeV (about 100 times heavier than a proton), the upper limit on the interactionprobability with an argon nucleus was 3.4 × 10⁻⁴⁵ cm². That's incredibly small: if you imagine an argon nucleus the size of a football field, a dark matter particle would have to hit it with millimeter precision to leave a trace.
Dust is the main enemy of such experiments. A single speck can generate hundreds of false signals, and scientists had to develop special methods to filter them out.
This result doesn't end the search, but it narrows the region where dark matter might be hiding. The next detectors will be even bigger and cleaner, and maybe one day the argon barrel will finally catch that elusive particle.