Scientists solve century-old riddle about origins of ‘ghostly’ subatomic particles

U of A physicists part of large international effort to find source 4 billion light years from Earth.


The first evidence of a source of high-energy cosmic neutrinos—ghostly subatomic particles that can travel unhindered for billions of light years from the most extreme environments in the Universe to Earth—has been revealed by an international team of scientists that includes a contingent from the University of Alberta.

The source has been traced back to a known blazar—a giant elliptical galaxy with a massive, rapidly spinning black hole at its core—situated in the night sky just off the left shoulder of the constellation Orion about 4 billion light years from Earth.

The blazar, designated by astronomers as TXS 0506+056, was first singled out following a neutrino alert sent by the IceCube Neutrino Observatory at the South Pole on Sept. 22, 2017.

“These intriguing results represent the remarkable culmination of thousands of human years of intensive activities by the IceCube Collaboration to bring the dream of neutrino astronomy to reality,” said U of A physics professor Darren Grant, IceCube spokesperson and Canada Research Chair in Astroparticle Physics, speaking of the international team made up of more than 300 scientists from 12 countries.

The observations help resolve a century-old riddle about what sends subatomic particles such as neutrinos and cosmic rays speeding through the universe.

Since they were first detected more than 100 years ago, cosmic rays—highly energetic particles that continuously rain down on Earth from space—have posed an enduring mystery: What creates and launches these particles across such vast distances? Where do they come from?

Because cosmic rays are charged particles, their paths cannot be traced directly back to their sources due to the powerful magnetic fields that fill space and warp their trajectories. But the powerful cosmic accelerators that produce them will also produce neutrinos, uncharged particles that are unaffected by even the most powerful magnetic field.

Because neutrinos rarely interact with matter and have almost no mass—hence their sobriquet “ghost particle”—neutrinos travel nearly undisturbed from their accelerators, giving scientists an almost direct pointer to their source.

The discovery unites two areas of research strength at the U of A, astrophysics and particle physics. Grant’s colleagues and fellow physics professors Claudio Kopper and Gregory Sivakoff, along with their graduate students, played instrumental roles in the discovery.

“This whole project is an interesting mix of scientists whose work together began through things like the IceCube Alerts and Astronomer’s Telegrams, and progressed to an impressive collaboration of facilities working together on one paper,” said Sivakoff. “We're witnessing the benefit of combining the talents of astrophysicists and particle physicists, combining not only photon detection but also new messengers such as astrophysical neutrinos, like the one announced in today’s discovery.”

Alexandra Tetarenko, Sivakoff’s PhD student, converted radio observations into some of the measured data reported in the primary publication in Science.

“It’s truly humbling to be part of such a revolution in the field. There will be some growing pains, but in the end, the best combination of the two will likely emerge to better shape our view of the Universe,” he said.

For Kopper’s part, as lead of the “diffuse neutrino flux” working group for IceCube, which is responsible for the detailed follow-up reconstruction system that provided the initial input used by all of the analyses for the discovery, he emphasized that time is most definitely of the essence when tracking something traveling billions of light years.

"We had to run detailed reconstructions of the neutrino event direction as fast as possible before sending the alert from IceCube to all of its partners,” said Kopper, adding the team was only able to react as quickly as they did because of access to a highly specialized computer cluster funded by the Canada Foundation for Innovation and made at the U of A. “It allowed us to accelerate the necessary computations."

In addition to Kopper and Grant, UAlberta physics professors Roger Moore and Juan Pablo Yáñez are also members of the IceCube collaboration. All told, U of A researchers account for four of the five scientists from Canadian universities.

The announcement for the landmark discovery, marked by two papers published in the prestigious peer-reviewed journal Science, was made by the international IceCube collaboration at the National Science Foundation in Washington. There are several follow-up observations detailed in a half dozen papers in addition to the two Science papers.