Signs of “neutrino fog” emerge, complicating searches for dark matter
- Scientists have long hypothesized dark matter as an invisible force explaining galaxy rotations, yet direct detection remains elusive.
- Now, highly sensitive detectors are picking up hints of high-energy solar neutrinos — known as the “neutrino fog” — which could overwhelm potential dark matter signals.
- This new interference complicates efforts to detect dark matter particles, though alternative models beyond WIMPs may still provide viable detection paths.
According to the theory favored by most scientists, the stars and galaxies that dot the night sky are just a small fraction of the mass and energy budget of the Universe. A form of matter called dark matter is thought to be five times more prevalent than ordinary matter. Over the past several decades, experiments searching for dark matter have increased in sensitivity over a million-fold but have found no direct evidence that it exists.
One complication in dark matter searches is that sufficiently sensitive dark matter detectors can also observe neutrinos. Neutrinos are subatomic particles that are most commonly created in nuclear reactions, and the biggest nuclear reactor around is the Sun. The Earth is deluged with solar neutrinos. When dark matter detectors become capable of seeing these messengers from the Sun, the signal will swamp any expected dark matter events — and this will greatly complicate future efforts to detect dark matter. In two separate papers (paper 1 and paper 2), two experimental groups are reporting that they are beginning to see solar neutrinos.
Solar neutrinos
Beginning as far back as the 1930s, with more sophisticated confirmation in the 1970s, astronomers have made an unexplained observation: Galaxies rotate more quickly than can be explained by visible matter and the known laws of physics. While several possible explanations have been considered, the research community has settled on one that seems to best fit the data. In this explanation, galaxies contain an invisible form of matter called dark matter. Dark matter contributes to the gravitational characteristics of galaxies, thus explaining their speedy rotation. However, as the name suggests, dark matter does not emit light, and therefore cannot be seen in telescopes.
While dark matter has not been directly observed, several different models of dark matter have been proposed, each with different properties. A common model is called a weakly interacting massive particle, or WIMP. WIMPs are electrically neutral and stable particles with masses from a few times that of a proton to perhaps 10,000 times the proton’s mass. In this model, each galaxy is surrounded by what one can imagine to be effectively a cloud of WIMPs. As planets orbit their respective stars, they move through the WIMP cloud. It is this motion that astronomers rely on in WIMP detection efforts.
Several different technologies are used to detect WIMPs. A common feature is that the detector is cooled to very low temperatures — low enough that the atoms and molecules in the detectors don’t move very quickly. If a WIMP particle moves through the apparatus, there is a chance that it will collide with an atom in the detector, causing it to recoil. Through a variety of means, the motion of the atom is detected, indirectly revealing the passage of dark matter.
Dark matter interactions are expected to be exceedingly rare, requiring that the detectors be shielded from outside interactions. They are made of low-radioactivity material and are frequently located deep underground, often in abandoned mines.
The “neutrino fog”
A complicating factor in dark matter searches is the existence of neutrinos, originating both from the Sun and high-energy cosmic protons that hit the Earth’s atmosphere. Although neutrinos interact very rarely, there are a ton of them: Every second, approximately 70 billion neutrinos from the Sun go through every square centimeter of the Earth’s surface. If only a tiny fraction of these neutrinos interacts in dark matter detectors, they will totally dwarf dark matter interactions. Researchers call this deluge of neutrinos “the neutrino fog.”
The neutrinos originating from the Sun come from a variety of different fusion processes. The most common comes from the fusion of two protons; however, this particular fusion process creates very low-energy neutrinos. However, there is a much rarer fusion process involving the element boron. This process accounts for only about 0.1% of solar neutrinos, but these neutrinos have much higher energy, making them easier to detect. These higher-energy neutrinos are called 8B neutrinos.
Two very sensitive dark matter detectors have recently published data that shows hints of the observation of 8B neutrinos. The first is called XENONnT, which uses 5.9 tons of liquid xenon, located in a laboratory deep under the Apennine Mountains in Gran Sasso, Italy. The second detector is called PandaX-4T. It also uses liquid xenon to try to detect dark matter and neutrinos. PandaX-4T is located in the Jinping Deep Underground Laboratory in Sichuan, China.
Both experimental collaborations use a particular process to search for 8B neutrinos, called “coherent elastic neutrino-nucleus scattering.” While neither detector firmly claims that they’ve observed the neutrino fog, both claim to have seen the first hints of 8B neutrinos. The significance of XENONnT’s claim is 2.73 standard deviations and an 8B neutrino flux of 4.7 x 106 neutrinos per square centimeter per second, with an uncertainty of +3.6 x 106 and -2.3 x 106. In contrast, PandaX-4T’s paper claims a 2.63 standard deviation significance, with an 8B neutrino flux of (8.4 ± 3.1) x 106 neutrinos per square centimeter per second.
Thus, the two experiments agree. Furthermore, the two measurements roughly agree with a more precise measurement made by a detector optimized to see 8B neutrinos. This more precise measurement was reported by researchers at the Sudbury Neutrino Observatory (SNO) and is approximately (2.5 ± 0.3) x 106 neutrinos per square centimeter per second. While the SNO apparatus could see neutrinos, it could not see dark matter.
The future of dark matter searches
It is standard in particle physics to require a minimum significance of 3 standard deviations to claim evidence for a phenomenon, and 5 standard deviations to claim observation, so neither experimental group has reached this level of statistical significance. However, the fact that both experimental groups are reporting similar results gives us reason to believe that the era will soon arrive in which dark matter experiments become sensitive to the neutrino fog.
This sensitivity will greatly complicate future dark matter searches for WIMPs. Researchers will need to do more sophisticated analyses to pull possible dark matter signals out of the noise, relying on measurements of the energy spectrum of observed dark matter candidates. WIMP searches will soon become far more difficult.
This impending observation of the neutrino fog will not make future searches for dark matter impossible. After all, there are other possible dark matter models that aren’t WIMPs. However, these new papers will certainly mean that the future of dark matter searches will become more interesting.
