Cosmic-ray mystery deepens at South Pole

A University of Alberta physicist has taken part in groundbreaking research into the origins of cosmic rays that may require a rethinking of current theories on the astrophysical phenomenon.

U of A particle physicist Darren Grant was part of an international research team that buried optical sensors deep under the South Pole in hopes of identifying an incredibly high-energy particle that originates in deep space and can pass through everything in its path—including the Antarctic ice sheet.

“We set the sensors in the ice to identify high-energy particles called neutrinos,” said Grant. The researchers were testing a theory that neutrinos are associated with gamma-ray bursts, a powerful astrophysical phenomenon originating in deep space that has the potential to shower Earth with high-energy particles.

“We tested 300 gamma-ray bursts which should have sent some neutrinos our way, but surprisingly we found none,” said Grant. “This result contradicts 15 years of predictions and challenges one of the leading theories for the origins of the highest-energy cosmic rays.”

The researchers say that either gamma-ray bursts are not responsible for the highest-energy cosmic rays, or the efficiency of neutrino production in the burst is much lower than has been predicted.

Grant says gamma-ray bursts happen about once a day with an explosion of light that can be seen halfway across the known universe. “If all the sun’s energy for 10 billion years were suddenly released in a 10-second blast, you would duplicate the power of a single gamma-ray burst.”

The Antarctic project, aptly named IceCube, is an international collaboration of 250 scientists from 10 countries including Canada. Grant was part of a team that buried more than 5,000 optical sensors under the ice, within a few hundred metres of the geographic South Pole.

“We drilled into the ice using a jet stream of near-boiling water and went down 2,500 metres,” said Grant. “The sensors were strung along cables lowered into the holes. We created a grid of sensor lines covering a square kilometre of the ice sheet.”

The researchers say that not finding neutrinos is a breakthrough because it lets them rule out a leading theory and helps them refine their search for answers.

"We have now added one important piece to the puzzle that, at least for the moment, has deepened the mystery,” said Grant. “This research program is a particle physicist’s playground.”

Grant and his U of A students were co-authors of a paper on the IceCube research that was published April 19 in the journal Nature.

Provided by University of Alberta