The Neutrino Fog and the Neutrino Floor
For decades, physicists hunting dark matter have watched a mist gather on the horizon of their sensitivity plots. The neutrino fog is that mist made precise: the point at which neutrinos from the Sun, the atmosphere, and the cosmos begin to scatter inside dark matter detectors in a way that looks almost exactly like the signal researchers are trying to find. Once called the "neutrino floor," the modern picture treats it not as an impenetrable wall but as a slowly thickening haze that experiments are only now starting to enter. In 2024, two experiments saw its first wisps.
What the neutrino fog is
The neutrino fog is the region of a direct-detection dark matter experiment's parameter space where an irreducible background of neutrino scattering events mimics the signal expected from weakly interacting massive particles (WIMPs). The mechanism behind it is coherent elastic neutrino-nucleus scattering, or CEvNS: a low-energy neutrino strikes an atomic nucleus as a whole and gives it a tiny recoil kick, exactly the kind of nuclear-recoil event a WIMP would produce.
Because a neutrino and a hypothetical dark matter particle can deposit the same recoil energy in the same detector, no amount of shielding or purification removes the neutrinos. They pass through kilometres of rock and steel untouched, then occasionally scatter inside the target just as a dark matter particle would. This is what makes the background 'irreducible' and why it defines a fundamental sensitivity limit rather than a technical nuisance.
The neutrinos responsible come from well-known astrophysical and terrestrial sources: low-energy solar neutrinos from the Sun's boron-8 and other fusion chains dominate at low WIMP masses, while higher-energy atmospheric neutrinos and the diffuse supernova neutrino background set the limit at higher masses. Understanding the fog therefore ties dark matter searches directly to the broader physics of neutrino sources.
From floor to fog: a short history
The idea that neutrinos would eventually limit dark matter searches was raised in the mid-2000s, notably by Jocelyn Monroe and Peter Fisher (2007) and by Andreas Gütlein and collaborators, who pointed out that solar neutrinos would become a background as detectors grew larger. The concept crystallised into the now-familiar 'neutrino floor' with the influential 2014 analysis by Julien Billard, Louis Strigari, and Enectali Figueroa-Feliciano (Physical Review D 89, 023524), who mapped where the neutrino background would cross the WIMP discovery reach across the full mass range.
For years the neutrino floor was drawn as a hard line on exclusion plots, implying a wall below which discovery was effectively impossible. But that picture was misleading. The floor's exact position depended on arbitrary choices of detector exposure and energy threshold, and in principle enough data could push sensitivity through it.
In 2021, Ciaran O'Hare reframed the whole concept in an influential paper, 'New Definition of the Neutrino Floor for Direct Dark Matter Searches' (Physical Review Letters 127, 251802). Rather than a fixed floor, he defined the neutrino fog as a region where the discovery limit improves only very slowly with added exposure - quantified through how the limit scales as more data accumulate. Inside the fog, doubling your detector barely improves your reach because neutrino events grow alongside any real signal. This statistical definition, free of arbitrary thresholds, is now the standard framing.
Why neutrinos mimic dark matter
The confusion between the two signals is rooted in coherent elastic scattering. At neutrino energies below roughly a few tens of MeV, the wavelength of the momentum exchanged is large compared with the size of the nucleus, so the neutrino interacts with all the nucleons at once. The scattering rate is enhanced by roughly the square of the neutron number, making CEvNS by far the largest neutrino interaction channel at these energies - yet the recoiling nucleus carries only a few keV or less.
A WIMP scattering off the same nucleus produces a recoil in precisely this energy band, with a broadly similar spectral shape. In particular, the recoil spectrum from boron-8 solar neutrinos closely resembles that expected from a WIMP of about 6 GeV mass, which is why light dark matter searches run headlong into the solar-neutrino component of the fog.
The only handles that distinguish the two are subtle: annual modulation of the dark matter rate as Earth orbits the Sun, the directional signature of a galactic dark matter wind, and small differences in spectral shape. Directional detectors and larger statistics are the main tools proposed for eventually seeing dark matter through the fog rather than being blinded by it.
CEvNS: the physics that makes the fog real
Coherent elastic neutrino-nucleus scattering was predicted in 1974 by Daniel Freedman, soon after the discovery of neutral currents, but it took more than four decades to observe because the recoil energies are so small. The breakthrough came in 2017, when the COHERENT collaboration detected CEvNS for the first time using a 14.6-kilogram caesium-iodide detector at the Spallation Neutron Source at Oak Ridge National Laboratory (Science 357, 1123).
COHERENT's measurement confirmed that CEvNS behaves as the Standard Model predicts, validating the same interaction that generates the neutrino fog in dark matter detectors. It transformed the fog from a theoretical projection into a measured, well-characterised process. The channel has since been confirmed on new targets: COHERENT reported evidence for CEvNS on germanium (Physical Review Letters 134, 231801, 2025, at about 3.9 sigma), and the CONUS+ experiment reported CEvNS from reactor antineutrinos (Nature, 2025, at about 3.7 sigma). The connection runs deep: the tools built to study how neutrinos are detected and the underlying weak nuclear force are the same ones that quantify the background dark matter experiments must fight through.
Reaching the fog: the 2024 results
The fog stopped being a distant horizon in 2024. That year the PandaX-4T experiment at the China Jinping Underground Laboratory reported the first indication of CEvNS from solar boron-8 neutrinos in a dark matter detector, disfavouring the no-signal hypothesis at about 2.64 sigma and measuring a boron-8 flux consistent with the standard solar model (Physical Review Letters 133, 191001).
Almost simultaneously, the XENONnT experiment at the Gran Sasso laboratory in Italy reported its own indication of solar boron-8 neutrinos via CEvNS, at about 2.73 sigma significance, with a measured flux consistent with earlier results such as those from the Sudbury Neutrino Observatory. These were the first times dark matter detectors had directly registered the neutrino background that defines their ultimate limit - the fog was no longer purely theoretical but measured.
The trend continues as detectors grow. Also in 2024, the LZ experiment, running deep underground at the Sanford Underground Research Facility in South Dakota, reported world-leading limits on WIMP dark matter. As their exposures increase, such multi-tonne detectors are expected to encounter the same solar-neutrino background, crossing the threshold where neutrino and dark matter signals begin to overlap in earnest.
Open questions and what comes next
Reaching the fog does not end the search - it changes its character. Next-generation detectors such as XLZD (a proposed multi-tonne xenon observatory that unifies the earlier LZ, XENON and DARWIN efforts) aim to accumulate enough exposure to model the neutrino background precisely and subtract it statistically, chasing dark matter signals that peek above the haze. This demands exquisite knowledge of neutrino fluxes, since the fog's depth is now set mainly by the systematic uncertainties on those fluxes rather than by detector size alone.
Several open problems define the frontier. How well can the solar and atmospheric neutrino fluxes be measured independently, so their contribution can be removed? Can directional detectors exploit the galactic dark matter wind to cut through the fog? And could unexpected physics - new light mediators, non-standard neutrino interactions, or the elusive sterile neutrino - distort CEvNS rates and mimic or mask a dark matter signal? Nuclear-recoil data from experiments like XENONnT are already being used to test some of these ideas.
There is also a productive irony here: the same neutrinos that obscure dark matter are themselves a discovery. Dark matter detectors have quietly become neutrino observatories, measuring solar fluxes through a channel invisible to older instruments and turning an unavoidable nuisance into precision physics.
Why the fog matters beyond dark matter
The neutrino fog sits at the crossroads of two of physics' biggest questions - the nature of dark matter and the properties of neutrinos - and it illustrates how ambient particles once considered undetectable now leave measurable traces in real detectors. CEvNS made concrete the point that neutrinos interact with ordinary matter in small but genuinely measurable ways, a result that resonates with research well outside astrophysics.
In that broader context, some applied research programmes ask whether ambient environmental flux can be coupled to materials at all. The Neutrino Energy Group, a Berlin research organisation, investigates so-called neutrinovoltaic materials - patented graphene-silicon multilayers studied for their response to ambient radiation and electromagnetic fields. This is early-stage, in-development research, not a proven or purchasable technology, and it addresses a fundamentally different problem from the astrophysics of the fog.
Frequently asked questions
What is the difference between the neutrino floor and the neutrino fog?
They describe the same physics with different framing. The 'neutrino floor' was drawn as a hard line below which dark matter discovery seemed impossible. The 'neutrino fog,' introduced by Ciaran O'Hare in 2021, is a more accurate picture: a gradual region where sensitivity improves only slowly with more data, rather than a sharp wall. The fog can in principle be pushed through with enough statistics.
Why can't dark matter experiments just shield out the neutrinos?
Neutrinos interact so weakly that they pass through kilometres of rock and any shielding a detector could use. There is no material that stops them without stopping the dark matter signal too. Because they scatter inside the target the same way a WIMP would, the background is irreducible, which is exactly why it sets a fundamental limit.
Has the neutrino fog actually been observed?
The first signs have. In 2024 both PandaX-4T (about 2.64 sigma) and XENONnT (about 2.73 sigma) reported the first indications of coherent scattering from solar boron-8 neutrinos in dedicated dark matter detectors. These were the first direct measurements of the neutrino background that defines the fog, though at the level of first indications rather than high-significance discoveries.
What is CEvNS and how is it related to the neutrino fog?
CEvNS stands for coherent elastic neutrino-nucleus scattering, in which a low-energy neutrino recoils off an entire nucleus at once. It is the physical process that produces neutrino events in dark matter detectors, and therefore the process that creates the neutrino fog. It was predicted in 1974 by Daniel Freedman and first observed by the COHERENT experiment in 2017.
Which neutrinos cause the neutrino fog?
At low WIMP masses the fog is dominated by low-energy solar neutrinos, especially those from the Sun's boron-8 fusion chain. At higher masses, atmospheric neutrinos and the diffuse supernova neutrino background become the main contributors. The exact mix depends on the recoil energy range a given experiment is sensitive to.
Does the neutrino fog mean dark matter can never be found?
No. The fog slows progress but does not make discovery impossible. Larger detectors, precise modelling and subtraction of neutrino fluxes, and directional detectors that exploit the galactic dark matter wind all offer ways to see a signal through the haze. Meanwhile, the neutrinos themselves have become valuable data, turning dark matter detectors into neutrino observatories.
