According to two studies published in Physical Review Letters, physicists in Italy and China have discovered glimmers of the ‘neutrino fog’, signals from neutrinos that match those believed to be created by dark matter.
Most neutrinos that stream through Earth are produced by fusion reactions in the Sun. Image Credit: NASA/Goddard/SDO
Nicole Bell, a theoretical physicist at the University of Melbourne in Australia, described the observations as a double-edged sword. On the one hand, they indicate that detectors have become sensitive enough to detect signals from dark matter, the enigmatic substance assumed to make up the majority of matter in the universe.
On the other hand, it implies that neutrino signals may mask the dark-matter signals that scientists are eager to observe.
Every second, billions of neutrinos pass through Earth without being detected due to their minimal interaction with regular matter. Fusion events in the Sun, such as the radioactive β-decay of boron-8, produce most of these almost massless particles.
According to Fei Gao, an experimental particle physicist at Tsinghua University in Beijing, dark-matter experiments are expected to detect the neutrino fog, also known as the neutrino floor. He works on the XENONnt dark matter experiment at the Gran Sasso National Laboratory near L'Aquila, Italy.
The neutrino fog is also intriguing because, according to Kate Scholberg, an experimental particle physicist at Duke University in Durham, North Carolina, measuring it demonstrates that dark-matter experiments can detect all “flavors” of neutrinos coming in from the Sun and even from nearby galaxies' exploding stars.
You could learn something about the total spectrum of all neutrinos.
Kate Scholberg, Experimental Particle Physicist, Duke University
Dedicated Search
Using liquid xenon detectors, the scientists at the China Jinping Underground Laboratory in Sichuan province conducted two years of XENONnT and PandaX-4T, another dark-matter experiment, to look for boron-8 solar neutrinos.
They concentrated on finding neutrinos that, through coherent elastic neutrino–nucleus scattering, struck the entire nucleus of xenon atoms rather than just their constituent pieces.
Signals from these events are strikingly similar to those produced if specific weakly interacting massive particle (WIMP) types, the most prominent candidate group for dark matter, collided with a xenon nucleus.
After applying various machine-learning approaches to analyze their data collection, the XENONnT researchers concluded that the detector had recorded 11 occurrences of solar neutrinos colliding with xenon nuclei1.
By lowering the experiment’s detection threshold to 0.33 kilo electronvolts, the PandaX-4T team was able to detect 75 of these events, which is an order of magnitude lower than most WIMP searches, according to team member Qing Lin, an experimental particle physicist at the University of Science and Technology of China in Hefei.
This increased the experiment’s sensitivity to even the smallest neutrino signals, but it also made the data set noisier than XENONnT’s as it contained more signals from electrons and radiation.
According to Bell, the results of both teams matched predictions of how solar neutrinos would interact with dark-matter detectors, but neither team's results had a statistical significance higher than 3σ, the standard for trustworthy evidence in physics. This implies that the neutrino fog is initially revealed by the observations.
“They have shown up exactly where we expected them to be,” Scholberg added.
Scholberg states that since solar neutrinos would only obscure certain forms of WIMP, it will be some time before signals from the neutrino fog obstruct studies looking for dark matter.
“We are just starting to reach the limits, but we are still far away from the end,” she said.
Scholberg states that since solar neutrinos would only obscure certain forms of WIMP, it will be some time before signals from the neutrino fog obstruct studies looking for dark matter.
According to Juan Collar, an experimental physicist at the University of Chicago in Illinois, neutrinos generated in Earth’s atmosphere rather than the Sun will be an even bigger problem for future dark-matter studies.
These atmospheric neutrinos will further complicate matters due to their ability to obfuscate signals from dark-matter particles of all masses. However, Collar asserts that much more needs to be discovered about the neutrino fog itself before it becomes a significant issue.
Journal References:
Aprile, E., et al. (2024) First Indication of Solar 8B Neutrinos via Coherent Elastic Neutrino-Nucleus Scattering with XENONnT Nature. https://doi.org/10.1103/PhysRevLett.133.191002
Bo, Z., et al. (2024) First Indication of Solar 8B Neutrinos through Coherent Elastic Neutrino-Nucleus Scattering in PandaX-4TNature. doi.org/10.1103/PhysRevLett.133.191001