Earth Can Absorb High-Energy Neutrinos, Physicists Find

Neutrinos are subatomic particles that are famous for passing through anything and everything. Now, physicists have demonstrated that our planet stops high-energy neutrinos — they do not go through everything. The experiment was achieved with the IceCube Observatory, an array of 5,160 basketball-sized sensors frozen deep within a km³ of very clear ice near the South Pole.

This image shows a visual representation of one of the highest-energy neutrino detections superimposed on a view of the IceCube Observatory at the South Pole. Image credit: IceCube Collaboration.

Neutrinos are among the most abundant particles in the cosmos. With almost no mass and no charge, they rarely interact with matter. Tens of trillions of neutrinos course through our bodies every second.

Theory predicts that at high energies — higher than can be generated by any earthbound particle accelerator — neutrinos can be expected to interact with matter and be absorbed in the Earth instead of continuing to transit the cosmos.

The first detections of extremely-high-energy neutrinos were made by the IceCube Observatory in 2013, but the mystery remained.

“We knew that lower-energy neutrinos pass through just about anything, but although we had expected higher-energy neutrinos to be different, no previous experiments had been able to demonstrate convincingly that higher-energy neutrinos could be stopped by anything,” said Penn State Professor Doug Cowen.

“We always say that no particle but the neutrino can go through the Earth,” added IceCube principal investigator Professor Francis Halzen, from the University of Wisconsin-Madison.

“However, the neutrino does have a tiny probability to interact, and this probability increases with energy. That probability is what scientists call the neutrino cross section.”

The new IceCube measurement determined the cross section for neutrino energies between 6.3 TeV and 980 TeV, energy levels more than an order of magnitude higher than previous measurements. The most energetic neutrinos studied so far from earthbound accelerators are at the 0.4 TeV energy level.

“IceCube’s sensors do not directly observe neutrinos, but instead measure flashes of blue light, known as Cherenkov radiation, emitted after a series of interactions involving fast-moving charged particles that are created when neutrinos interact with the ice,” the physicists said.

“By measuring the light patterns from these interactions in or near the detector array, IceCube can estimate the neutrinos’ energies and directions of travel.”

Analyzing a year of IceCube data gathered between May 2010 and May 2011, they put 10,800 neutrino interactions under the microscope.

“We looked mostly at neutrinos created when high-energy cosmic rays crash into the nuclei of nitrogen or oxygen in the Earth’s atmosphere. Those collisions produce a cascade of subatomic particles that can generate neutrinos,” they said.

“The sample also included a smaller number of neutrinos probably created in yet-to-be identified cosmic accelerators such as black holes.”

“We found that fewer of the most energetic neutrinos were making it to the detector from the northern hemisphere, where the particles would have to transit the entire Earth, including the dense core of our planet, before reaching the IceCube sensors. From less obstructed, near horizontal trajectories, more neutrinos were detected.”

The IceCube measurements conform to the Standard Model of particle physics.

“In the absence of new physics, the Standard Model allows us to calculate the neutrino-proton cross section at the energies probed by IceCube,” Professor Halzen said.

“What we measure is consistent — up to now — with what is expected. We were of course hoping for some new physics to appear, but we unfortunately find that the Standard Model, as usual, withstands the test.”

“However, the advantage of IceCube is its ability to measure the highest energy neutrinos, which are produced in cosmic accelerators — supermassive black holes, the violent hearts of star-forming galaxies, and galaxy clusters — that no accelerator on Earth can match,” he added.

“If, for example, IceCube data harbor evidence of neutrinos with cross sections greater than what scientists have calculated using the Standard Model, it could invoke new physics such as compact, hidden spatial dimensions.”

The results appear in the journal Nature.

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M.G. Aartsen et al (IceCube Collaboration). Measurement of the multi-TeV neutrino interaction cross-section with IceCube using Earth absorption. Nature, published online November 22, 2017; doi: 10.1038/nature24459