By Meghan Chua
Deep in the ice beneath the South Pole, an array of sensors in the IceCube detector picked up on something in September 2017 that hinted at a solution to a centuries-old mystery.
In the following months, a team of international scientists scrutinized everything they knew about the cosmic event. They arrived at the conclusion that the subatomic, ghostly particle called a neutrino that entered the detector had come from a specific type of galaxy, far away from Earth.
In two papers published in Science in July 2018, the researchers detail the first tangible evidence that the galaxy, called a blazar, is not only the source of the high-energy neutrinos detected in September but also a likely source of a mysterious high-energy phenomenon known as cosmic rays.
Those papers include more than a dozen UW–Madison graduate students involved in the study as authors. The IceCube lab, supported by the National Science Foundation, is operated by UW–Madison with collaborators spanning 50 institutions in 12 countries. On campus, the Wisconsin IceCube Particle Astrophysics Center (WIPAC) connects researchers, faculty, and students whose work focuses on neutrino astronomy with IceCube.
Physics graduate student Ibrahim Safa joined WIPAC in June 2017, a few months before the alert that IceCube’s detector had picked up an interesting event. He studies high-energy particle physics and astrophysics, developing new methods to analyze decades’ worth of data in a search for answers about our universe.
“I was tinkering with other things when that happened,” Safa recalls. “It kind of shifted where my analysis is going, based off what we saw. Since then, it’s been crazy. Everyone’s interested now.”
Researchers spent the months between September 2017 and July 2018 checking, double-checking, and triple-checking the data supporting their conclusion. There wasn’t a lot of time for him to reflect on how significant their findings would be, Safa said.
He only fully realized how much attention it would capture once the story went public. In December, Science recognized the discovery as a runner-up for its 2018 Breakthrough of the Year.
The discovery also gives Safa new material to aid him in his work. Now that the team has a better idea of how neutrino emissions happen, he can look at past data and build models to test for things that may have been overlooked in the past. It’s been an exciting time to be a physicist, Safa said.
“Physics has a lot of times when you’re just basically blind in the dark and you’re just trying to grab a hold of something,” he said. “There are so many wrong answers and very few right answers, so when you actually find one, that’s really exciting. I would say I’m very motivated.”
Physics graduate student Raamis Hussain agreed that the discovery raised excitement among those working on detecting neutrinos, whether or not they were involved in the discovery directly. His research uses IceCube data to identify transient sources, such as the merging of neutron stars, that produce neutrinos for a short time period.
“It’s a novel and exciting area of research which gives us the ability to see the universe in a way that has only become possible in the past decade or so,” Hussain said. “We can study objects that traditional astronomy cannot probe.”
Sarah Mancina, also a physics graduate student at WIPAC, said learning about the sources of neutrinos and cosmic rays helps scientists understand radiation, informing future possibilities such as deep space travel. The earth’s atmosphere acts as a shield against this cosmic radiation, but in space, the radiation is a constant bombardment. Still, Mancina says that’s not the only reason to study neutrinos.
“There’s also just pure curiosity of trying to understanding our universe. [Neutrinos] are these interesting particles because they sort of just travel through stuff,” she said. “They don’t interact electromagnetically, which is how we experience the world.”
Mancina looks for starting tracks, which are lines of light that that suddenly appear in the middle of a detector and could represent neutrinos. Her work is based on an algorithm she’s been building and refining to determine whether the detector did, in fact, pick up a neutrino, or if it was any number of other things. She searches the data from IceCube, as well as other telescopes around the world, to find significant events that could tell researchers more about neutrinos.
She said that physicists find it easy to wonder whether they’re ever going to observe a significant event. The excitement around discovering the source of the neutrino that IceCube detected challenged that doubt.
“This discovery, I think, brought back some optimism. It made me excited, because it’s like, ‘oh, maybe I could see something’,” she said.
Read more about the discovery at news.wisc.edu/cosmic-rays/.