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- The LUX-ZEPLIN experiment in South Dakota recorded a single anomalous pulse on June 16th 2023.
- The team says it carries every characteristic of a WIMP at least 200 times heavier than a proton.
- It appeared just once, below the confidence level physicists require for a discovery — so there is no verdict yet.
A peculiar pulse turned up in the instrumentation of the LUX-ZEPLIN (LZ) experiment, a subterranean dark matter search in South Dakota (pictured), on June 16th 2023. That merits attention. For nearly a hundred years, researchers have sought dark matter without success. Presenting for the collaboration at the TeV Particle Astrophysics conference on September 1st, Sam Eriksen of the University of Bristol said the team has grounds to suspect the pulse might represent a “weakly interacting massive particle” (WIMP), a long-favoured contender for the cosmos’s missing matter.
What the LUX-ZEPLIN experiment saw
Since 2021, the LZ project has pursued WIMPs with a reservoir holding seven tonnes of liquefied xenon. When particles traverse this reservoir they strike xenon atoms and produce bursts of illumination. Ordinary matter tends to bounce off the electrons of xenon, whereas WIMPs are expected to rebound from its nucleus. Such a “nuclear recoil” gives off distinctive light patterns. Dr Eriksen says the bursts observed by the group in 2023 display every characteristic of a WIMP at least 200 times heavier than a proton.
Estimated share of all mass in the universe
Liquefied xenon in the LZ reservoir
Minimum mass of the candidate WIMP, against a proton
Why physicists still favour WIMPs
Researchers feel confident that unseen matter is real. Their finest estimates indicate it constitutes about 85% of all mass in the universe and performs several crucial functions, including holding galaxies together. Yet its actual composition remains completely unknown. It neither gives off, soaks up nor bounces back light or any other electromagnetic radiation, and reveals itself solely through gravitational influence.
WIMP — a “weakly interacting massive particle”. A conjectured hefty, slow-moving speck that would engage with ordinary matter solely through gravity and the weak nuclear force, and that may have roamed freely shortly after the Big Bang.
Across the decades, theorists have proposed a varied lineup of possible culprits for these effects. WIMPs rank among the strongest leads. These conjectured hefty, slow-moving specks would engage with ordinary matter solely via gravity plus an obscure interaction known as the weak nuclear force. According to the idea, such specks may have roamed freely shortly after the Big Bang. Then, as the cosmos grew larger and chillier, their quantity locked in at precisely the level needed to account for the present-day density of dark matter.
Why this is not a discovery — yet
No verdict has been reached. The candidate WIMP appeared just a single time and at a confidence level below the threshold physicists require for a genuine discovery. Still, Dr Eriksen says, no WIMP-search effort has ever recorded an event on this scale before. And for a quarry as hard to catch as dark matter, that alone certainly deserves notice.
One pulse is not a particle. The 2023 event arrived at a confidence level below the threshold physicists require for a genuine discovery, so the LZ collaboration is treating it as a lead to be tested, not a result.
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