High Energy Neutrino Source Discovered, Research Heralds ‘New Era’ for Particle Physics
Originally published on Newsweek
Deep below the surface of Antarctica, scientists are hunting for ghosts. Tiny particles called “neutrinos” fill the universe, but they’re almost impossible to detect. Their mysterious origins have eluded researchers for decades.
Now, in a discovery that heralds a “new era” of neutrino research, physicists have finally pinned down a source. Researchers probing a huge cube of ice at the South Pole think a violent “blazar” galaxy is churning out the stealthy specks. They reported their results in two papers published in the journal Science.
The discovery also sheds light on a mystery more than 100 years in the making, helping scientists understand the cosmic rays that radiate throughout the universe.
“If you stick your thumb out right now, there are one billion neutrinos going through your thumbnail every second. They are going through your [nail], through your body, through everything,” University College London particle physicist Linda Cremonesi told Newsweek.
But these ghostly particles almost never interact with matter.
“Every second we have something like 100 trillion neutrinos going through our body, but in our lifetime we will on average interact with just one of them,” said Cremonesi, who was not involved in the research.
This stealthy travel makes neutrinos the perfect scouts through which to spy on the deepest, darkest parts of the universe. While a single sheet of paper can absorb light’s photons, Cremonesi explained, neutrinos can travel undetected for eons.
That’s the reason this discovery may signal “the beginning of a new era in neutrino astronomy,” she added. But it’s also why the particles are so hard to find.
“The core difficulty is always the same for everything related to neutrinos: They almost never interact with matter,” Chad Finley from Stockholm University told Newsweek. “The vast majority [of neutrinos] that arrive at Earth will pass through without leaving a trace. This has earned them the name ‘ghost particles.’”
The IceCube facility at the South Pole hunts for neutrinos like a telescope detects light. But instead of pointing to space, its sensors are buried in a clear chunk of ice buried deep underground.
Thought to be forged in the churning bellies of nuclear reactors like the Sun, the phantasmagoric specks pour out into the universe—abundant but solitary. Only a small portion will likely ever interact with matter, Finley explained.
IceCube hunts for “high energy” neutrinos, where a single particle has almost a micro-joule of energy, IceCube researcher Naoko Kurahashi Neilson from Drexel University told Newsweek. The most energetic neutrinos should shed light on the highest-energy astrophysical processes.
“When a high energy neutrino collides with an atom, a shower of subatomic particles explodes forth in the same direction,” said Finley, who is also an IceCube researcher.
This interaction creates a faint blue light that passes through the clear, pristine ice. IceCube’s sensors are spread out across about a quarter of a cubic mile, so they’re watching about a billion tons worth of hydrogen and oxygen atoms just waiting to bump a neutrino or two, Finley explained.
Millions to billions of particles travel through IceCube, so a team of top scientists work to filter out the neutrinos from this haze, Kurahashi Neilson said. Some 300 researchers from 49 institutions in 12 countries are involved in this chilly hunt.
Although IceCube had detected neutrinos before, an event observed on September 22, 2017 finally pointed scientists towards a specific source. This neutrino lined up with a bright object sitting four billion light years away—a blazar called TXS 0506+056.
Blazars—galaxies surrounding hungry black holes that guzzle on orbiting matter and spurt out plasma jets in the direction of Earth—are some of the most energetic phenomena in existence.
When the team looked back at their research from 2014-2015, they matched up more neutrino events with the same source. This made them pretty confident that—eons ago—the neutrinos had burst from the distant blazar, before slowly making their way to Earth. “We’ve calculated that there’s only a 0.02% chance this is just coincidence,” Kurahashi Neilson said.
As well as blazars and stars like the Sun, scientists think neutrinos might also come from gamma ray bursts, supernovae remnants, starburst galaxies and other highly energetic astronomical phenomena. But, so far, researchers haven’t linked any of the mysterious particles to these sources.
If the blazar discovery wasn’t revolutionary enough, the IceCube research also advances scientists’ understanding of cosmic rays—high-energy radiation that seeps into our solar system. “[Neutrinos are thought to come from high energy particle collisions where cosmic rays are created. But the sources of high energy cosmic rays are themselves unknown,” Finley explained. “This has been one of the biggest unsolved questions in astrophysics for decades.”
The new research suggests that blazars might also be a source of this very high energy cosmic radiation.
Plenty of other neutrino mysteries still await the IceCube Collaboration. Scientists are keen to find out if other blazars are pumping out the particles, and where else the stealthy specks are spurting from.
“This measurement really launches the field of neutrino astronomy,” IceCube researcher Darren Grant from the University of Alberta told Newsweek.
“All of astronomy since the dawn of time has been using light. We know where stars are, where galaxies are because we see them in photons,” Kurahashi Neilson said. “Now we want to see what the universe looks like in neutrinos.”