Using a gigantic particle detector buried deep in ice near the South Pole, an international team of researchers has measured how the Earth absorbs very high-energy neutrinos.
“They behave exactly as predicted by the Standard Model,” says physicist Gary Hill of the University of Adelaide, one of the authors of the study. “It turns out to work beautifully.”
Neutrinos are incredibly tiny particles that usually interact only weakly with matter. Although the universe is so swamped with neutrinos that around 100 million of them stream through your body every second, it is relatively rare that they hit anything. This makes them hard to detect.
The IceCube Neutrino Observatory watches for these rare collisions using a kilometre-a-side cube composed of thousands of sensors embedded deep within the Antarctic ice. The sensors look out for flashes of blue light given off by charged particles that are created when incoming neutrinos interact with the ice. Tracking the flashes of light allows scientists to trace back the direction the neutrino came from and work out how fast was moving.
The Standard Model of particle physics, which sums up our current best knowledge of how the universe works at the subatomic level, predicts that neutrinos will become less aloof at higher energies.
Until now, however, physicists have been unable to check this prediction against experiments, as most other neutrino detectors only measure the lower-energy neutrinos emitted by the Sun or particle accelerators.
IceCube, however, can observe extremely high-energy neutrinos which are produced when fast-moving cosmic rays produced in space interact with atoms in Earth’s atmosphere.
By studying more than 10,000 such neutrinos observed in 2010 and 2011, the researchers were able to confirm that very few of them were coming from paths that had travelled through most of the Earth. This led them to the conclusion that those high-energy neutrinos must have been absorbed during their passage through the planet.
“We were of course hoping for some new physics to appear,” says Francis Halzen of the University of Wisconsin-Madison, who leads the IceCube collaboration. “But we unfortunately find that the Standard Model, as usual, withstands the test.”