Dark matter hunt: WIMP detectors enter the neutrino fog

Under mountains in the Apennines, beneath the Jinping Mountains of Sichuan, and deep in a South Dakota mine, massive liquid-xenon detectors built to find WIMPs are now seeing a different signal: neutrinos. The detectors’ increasing size and sensitivity have begun to expose them to the so-called "neutrino fog," where solar and stellar neutrinos produce background events that can drown any WIMP signature.

How did WIMP detectors end up in the neutrino fog?

The detectors grew so large and sensitive that neutrino interactions with xenon, while rare, now mimic the infrequent blips a WIMP would make. Physicists knew the neutrino background existed for decades, but the latest generation of liquid-xenon experiments has reached a sensitivity at which neutrinos are unavoidable; neutrinos cannot be shielded because they slip through the Earth itself. That reality means some current detectors, including the LZ experiment at the former Homestake Mine in South Dakota, are probing a regime where distinguishing a WIMP signal from neutrino events becomes extremely difficult.

What alternatives are physicists pursuing instead?

Researchers are broadening searches across many mass and interaction ranges, with axions and low-mass dark matter getting renewed emphasis alongside novel technologies like quantum sensors and liquid-helium detectors. Axions were proposed in the 1970s and would be far lighter than an electron—about a trillionth to a millionth the electron’s mass—and require ultracold, magnetized "radio" chambers to convert them into detectable photons. The first full-size haloscope was built at Lawrence Livermore National Laboratory in 1994, and today experiments such as MADMAX and ABRACADABRA probe this space. So far, physicists have scanned between 10% and 20% of the parameter space for axions that would solve the strong CP problem; many teams are also targeting axion-like models that could still serve as dark matter even if they do not solve that other puzzle.

Could one final giant xenon detector still find WIMPs?

A proposed follow-up called XLZD would have used 60 to 80 metric tons of liquid xenon—about the yearly global production of xenon and at least six times more xenon than the biggest current detector contains—but its future is in doubt after a policy decision. At a particle physics meeting in December 2025, the US Department of Energy announced the US would neither host XLZD nor pay its share of the price tag, which could be well over $300 million. That withdrawal may have effectively killed the project, as one experimentalist warned, meaning the next experiment using this liquid-xenon WIMP approach might be the last attempt at pushing through the neutrino fog.

Why it matters

The change forces a reallocation of experimental effort and funding toward a much wider search strategy. The WIMP program dominated dark matter thinking for decades—born in the 1980s from supersymmetry-inspired ideas and reinforced by the hope that the Large Hadron Collider would find new particles after it turned on in 2008—but the combination of null collider results and detectors hitting the neutrino floor has opened many alternatives. New techniques are now technologically plausible, and experimentalists are optimistic: as one leader put it, there is "a great deal of excitement" and finally the instrumentation to pursue these ideas.

What to watch

Whether XLZD is revived by new international partners or quietly dies will be the clearest near-term signal for the fate of large xenon searches. Track US funding decisions tied to the December 2025 announcement and progress reports from axion programs as they move beyond the current 10%–20% coverage of the parameter space. Successes or limits from those axion and low-mass experiments will determine if the community shifts permanently away from the WIMP paradigm or finds new motivation to push past the neutrino fog.