Year in review: Neutrinos leave tracks in ice

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In the dark depths of an Antarctic glacier, flashes of light triggered by wispy particles called neutrinos are providing rare clues about the universe’s most extreme environments. After discovering the first high-energy neutrinos from beyond the solar system late last year, researchers with the IceCube Neutrino Observatory spent 2014 tracing the particles’ origins to the locations of the mysterious violent objects that produced them.

“For the first time, I can point to an area in the sky and say there’s an ultrahigh-energy object there,” says IceCube astrophysicist Nathan Whitehorn of the University of California, Berkeley. The next step, he says, is using the neutrino data to identify those objects.

For decades, scientists have puzzled over the origins of speedy charged subatomic particles, some of which pelt Earth with about 100 million times the energy of particles whizzing around the best human-made accelerator. Even the most powerful gamma-ray burst should not be able to accelerate particles to those speeds. Scientists can’t trace the particles back to their sources because magnetic fields skew the particles’ paths.

But high-energy neutrinos, which also form near the universe’s most violent objects, travel in a straight line. Neutrinos have no charge, so they are unaffected by magnetic fields and pass through gas clouds and galaxies unperturbed.

Since 2010, IceCube has hunted for cosmic neutrinos beneath the surface of the Antarctic ice sheet. Nearly 5,500 sensors spread over a cubic kilometer detect flashes of light when neutrinos hit atoms in the ice. IceCube has detected at least 37 high-energy neutrinos from beyond the solar system (SN Online: 4/7/14).

In unpublished work this year, IceCube scientists mapped the birthplaces of a subset of neutrinos. The collision of an atom with one type of neutrino produces a muon, which physicists can track as it burrows through the ice. Researchers used each muon track to trace the path of the parent neutrino and pinpoint the area of the sky from which it arrived.

Preliminary data reveal that high-energy neutrinos bombard the planet from all directions evenly, suggesting that the particles are produced by many distant objects rather than a few nearby ones (SN: 5/17/14, p. 8).