Intergalactic Particles Spotted at South Pole

October 23, 2015 | Sarah Tse

Aurora borealis above the IceCube Lab at the South Pole.
Photo credit: IceCube Collaboration

Scientists have confirmed the cosmic origins of mysterious particles that can pass through miles of Antarctic ice without a scratch.

When a star collapses into a supernova, the core becomes so hot and dense that it unleashes an outburst of high-energy, low-mass particles called neutrinos. Neutrinos are so nimble and slight that they can pass through all kinds of cosmic obstacles—including stars, planets, magnetic fields, and clouds of interstellar dust—to arrive at Earth in much the same condition as when they first formed.

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Millions of neutrinos bombard Earth on a daily basis, flitting through its dense matter like phantoms and continuing on through space. But despite their abundance, neutrinos are exceedingly difficult to study, partially because they leave almost no trace, and partially because they get drowned out by a flood of other types of subatomic particles. Recently, scientists succeeded in detecting the cosmic particles at the IceCube Neutrino Observatory. The Observatory has a cubic-kilometer-sized detector with thousands of optical sensors, buried 8,000 feet beneath the South Pole.

Neutrinos themselves, being nearly massless and uncharged, leave almost no evidence of their presence. On the rare occasion that they meet head-on with other particles, they spawn secondary particles called muons that can flash through solids—ice in this case—at the speed of light. The detector can sense these muons by picking up the ripples of light they leave in their wake, called Cherenkov radiation. The waves of radiation also reveal the direction the neutrinos were travelling in.

To single out the muons derived from true cosmic neutrinos, the detector points down into the ground towards the North Pole, to receive signals from the Northern Hemisphere’s sky. This way, the Earth’s mass acts as a filter to block out the background noise of muons created from other sources—when cosmic rays clash into the planet’s atmosphere, for example.

“Looking for muon neutrinos reaching the detector through the Earth is the way IceCube was supposed to do neutrino astronomy and it has delivered,” said Professor Halzen, the principal investigator of IceCube, in a statement from the University of Wisconsin-Madison.

Physicists already knew that neutrinos can form from local sources, such as the sun. When they first discovered the influx of neutrinos in 2013, they presumed most of those neutrinos formed within our solar system, or perhaps somewhere nearby in the Milky Way. But this recent study found that 21 of those initially detected neutrino-related events had high enough energy levels to hint at origins beyond our galaxy. These muon neutrinos came at Earth from all different directions at similar rates, which indicates they formed from high-energy phenomena outside the Milky Way—perhaps even as long ago as the Big Bang.

However, the random trajectories of each muon neutrino prevents scientists from determining any single point source. In addition to a star’s collapse into a supernova, other naturally-made accelerators can generate neutrinos; for example, when galaxies forge new stars at rapid speeds, or when a black hole at the center of a galaxy agitates atoms into higher-energy collisions. Further analysis of the neutrinos’ voyages will point the way to these high-energy cosmic phenomena, and may answer deep questions about the physics of the universe.

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