Antarctic detector offers first look at how Earth stops high-energy neutrinos in their tracks
(22 November 2017 - National Science Foundation) An interdisciplinary team of researchers using the IceCube Neutrino Observatory in Antarctica has measured how certain high-energy neutrinos are absorbed by the Earth, as opposed to passing through matter as most neutrinos do. The finding could help expand scientists' understanding of the fundamental forces of the universe.
Funded and managed by the National Science Foundation (NSF), the IceCube Neutrino Observatory conducts research into these nearly massless particles.
The IceCube Neutrino Observatory at NSF's Amundsen-Scott South Pole Station. (courtesy: Mike Lucibella, Antarctic Sun)
"IceCube was built to explore the frontiers of physics and, in doing so, possibly to challenge existing perceptions of the nature of the universe," said James Whitmore, program director in NSF's Division of Physics. "This new finding and others yet to come are in that spirt of scientific discovery. IceCube is truly a remarkable window on the universe."
Scientists with the international IceCube Collaboration, which includes more than 300 researchers from 48 institutions in the U.S. and 11 other countries, say in a paper published in the journal Nature how some very energetic neutrinos from space interact with matter and are absorbed by the Earth.
Neutrinos are subatomic particles, most of which pass through anything and everything, only very rarely interacting with matter. In contrast to the newly discovered properties of the high-energy neutrinos, about 100 trillion neutrinos with lower energies pass through the human body, on average, every second without being absorbed.
"Neutrinos have quite a well-earned reputation of surprising us with their behavior," says Darren Grant, spokesperson for the IceCube Collaboration and a professor of physics at the University of Alberta in Canada. "It is incredibly exciting to see this first measurement and the potential it holds for future precision tests."
For this study, the collaboration included geologists who have created models of the Earth's interior from seismic studies as part of a larger multidisciplinary team. Physicists worked with the geologists to measure how neutrinos are absorbed by the Earth. A deeper understanding of how often a neutrino will come through the Earth to eventually interact within the IceCube detector also requires detailed knowledge of the Antarctic ice properties, the interaction of cosmic rays with the Earth's atmosphere, and how neutrinos interact with matter.
IceCube is an array of 5,160 optical sensors, each roughly two feet in diameter, deeply encased within a cubic kilometer of very clear Antarctic ice near NSF's Amundsen-Scott South Pole Station. IceCube's sensors do not directly observe neutrinos. Instead, they measure flashes of blue light, known as Cherenkov radiation, produced by muons and other fast-moving charged particles created when neutrinos interact with the ice. By measuring the light patterns from these interactions in or near the detector array, IceCube can estimate the neutrinos' directions and energies.
NSF's Office of Polar Programs and Division of Physics support the management and operations of the observatory. IceCube was built with funding from an NSF Major Research and Equipment and Facilities Construction award, with assistance from partner funding agencies worldwide. The University of Wisconsin-Madison is the lead institution for the IceCube Collaboration, coordinating data-taking and management and operation. As manager of the U.S Antarctic Program, NSF operates three year-round stations in Antarctica, including Amundsen-Scott.
The IceCube Collaboration research team found that fewer energetic neutrinos made it to IceCube's detector on paths that took them all the way through the Earth than from less obstructed paths, including near-horizontal trajectories. The probability of neutrinos being absorbed by the Earth was consistent with expectations from the Standard Model of particle physics, a theory that scientists use to explain the fundamental forces and particles in the universe. This probability -- that neutrinos of a given energy will interact with matter -- is what physicists refer to as a "cross section."
"Understanding how neutrinos interact is key to the operation of IceCube," said Francis Halzen, principal investigator for the IceCube and a professor of physics at the University of Wisconsin-Madison.
Precision measurements at the HERA particle accelerator complex in Hamburg, Germany, provided a foundation to calculate the neutrino cross sections, which would apply to IceCube neutrinos of very high energies if the Standard Model is valid at these energies.
"We were of course hoping for some new physics to appear, but we unfortunately find that the Standard Model, as usual, withstands the test," Halzen says.
This study provides the first cross-section measurements for a neutrino energy range that is up to 1,000 times higher than previous measurements at particle accelerators. Most of the neutrinos studied by the research team were more than a million times more energetic than the those produced by sources like the sun or nuclear power plants.
In addition to providing the first measurement of the Earth's absorption of neutrinos, the researchers' analysis shows that IceCube's scientific reach now extends beyond the observatory's core focus on particle physics discoveries and the emerging field of neutrino astronomy. Its work could also have applications in the fields of planetary science and nuclear physics. The team's analysis will be of interest to geophysicists seeking to use neutrinos to image the Earth's interior, although such work would require more data than the current study used.
The neutrino-interaction events selected for the study have energies of at least one trillion electron volts, or 1 teraelectronvolt (TeV), roughly the energy of motion of a flying mosquito. At this energy, the Earth's absorption of neutrinos is relatively small; the lowest-energy neutrinos in the study largely served as a baseline measurement for neutrinos that the Earth did not absorb.
The analysis was sensitive to absorption in the energy range from 6.3 TeV to 980 TeV.. At these energy levels, each individual proton or neutron in a nucleus acts independently, so the probability of absorption by the Earth depends on the number of protons or neutrons that each neutrino encounters. The Earth's core is particularly dense, so absorption is largest there.
By comparison, the most energetic neutrinos studied at particle accelerator facilities were at energies below 0.4 TeV. Researchers have used accelerators to fire beams containing an enormous number of lower energy neutrinos at detectors, but only a very tiny fraction yield interactions.
IceCube researchers used data collected from May 2010 to May 2011, from a partial array of 79 "strings," detector units embedded more than a mile deep in the ice, each containing 60 sensors. Researchers compared data to a model describing how neutrinos propagate through the Earth to find the cross section that best fits the data. Simulations to support the analysis have been conducted using supercomputers at the University of Wisconsin-Madison and at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC).
Physicists now hope to repeat the study using an expanded, multiyear analysis of data from the full 86-string IceCube array and look at higher ranges of neutrino energies for any hints of new physics beyond the Standard Model. IceCube has already detected multiple ultra-high-energy neutrinos, in the range of petaelectronvolts (PeV), which have energy levels 1,000 times higher than those detected in the TeV range.
More data will both reduce researchers' uncertainties and generate findings about neutrinos at even higher energies, opening new opportunities to explore nuclear effects in the Earth and collective magnetic effects. With a better understanding of nuclear neutrino interactions, scientists hope to explore the boundary between the Earth's inner solid core and its liquid outer