Buried Antarctic observatory clocks neutrino changes

New measurements narrow possibilities for elusive particles. Phil Dooley reports.

The IceCube Observatory at the South Pole.
The IceCube Observatory at the South Pole.
Courtesy of IceCube Neutrino Observatory

A giant matrix of detectors buried deep in the ice in Antarctica has produced another clue in the quest to understand the cosmos’s most mysterious particles, neutrinos.

Neutrinos are the smallest known elementary particles, more than a million times tinier than an electron. They are the prime target of an installation called the IceCube Observatory, a network of more than 5000 detectors that spans a cubic kilometre of ice near the South Pole. It has been operating since 2010.

The detectors, some positioned more than two kilometres deep, are the only ones on Earth engineered to measure high energy types of neutrinos. However, just like less sensitive versions around the globe, they pick up only a tiny fraction of the trillions that pass every second. Most go straight through.

Neutrinos come in three types, or flavours: muon, tau and electron. They are so hard to observe that nearly 90 years after their discovery physicists still aren’t sure of their masses, or exactly how those masses relate to each other.

The new results from IceCube, published in the journal Physical Review Letters, are another piece in this jigsaw puzzle: a range of measurements at energy levels 10 times higher than those achieved by earlier tests. The results narrow the range of possible activity for the particles.

Strangely, neutrinos morph from one flavour to another as they travel, a process known as oscillating. It is one of these flavour changes measured by scientists led by Tyce DeYoung of Michigan State University in the US.

The team looked at how neutrinos exhibiting muon flavour turn into tau, and then turn back again, a dynamic related to the difference between their masses.

Muon neutrinos are produced around the Earth by cosmic rays hitting the atmosphere. Randomly, some of them fly towards the South Pole, passing straight through the ground, oscillating as they travel.

Some originate just a few kilometres above IceCube, and reach the detector without changing. The furthest hail from the North Pole, 13,000 kilometres away.

Neutrinos oscillate slower at higher energies, so for the high-energy neutrinos in this experiment (recorded at between 5.6 and 56 gigaelectronvolts), distances the size of Earth were needed for the changes to occur, says IceCube scientist, Gary Hill, from Adelaide University in Australia.

“At certain energies, you are seeing the first oscillation point of the neutrinos. If they go even further, they'll oscillate back again,” he says.

“We see the sweet spot in distance-to-energy ratio – for each different distance, there's some energy where the probability to have oscillated is a maximum.”

The measurements matched earlier low-energy results, confirming the approach works.

“There's no real reason to suspect that we wouldn't have found the same result, but it's always good to see the underlying theory works over more than one energy range,” he adds.

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Phil Dooley is a freelance science writer based in Canberra.
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