What is a neutrino anyway? A look at the science behind this year's Nobel Prize in Physics

If you wanted a Nobel Prize, you would do well to study neutrinos. To date, the Nobel Prize in Physics has been awarded five times to research related to neutrino detection and discovery. This year, Francis Halzen earned the Nobel "for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin."

University of California San Diego distinguished professor of physics George Fuller specializes in theoretical nuclear and elementary particle astrophysics and cosmology and worked with Halzen on an early paper on IceCube. He is uniquely positioned to answer two questions: What are neutrinos? And how does an icy observatory at the South Pole help us learn about them?

What are neutrinos?

Neutrinos are elementary particles, meaning they cannot be broken into smaller components, as far as we know. There are three neutrino "flavors": electron, muon and tau. Unlike some other elementary particles, neutrinos have no electric charge. Although they have mass, they are incredibly light—maybe 10 million times lighter than an electron. Scientists don't actually know the mass of neutrinos, and they don't know where the mass comes from either. Because they interact weakly with matter, they can move through space nearly at the speed of light.

Some come from the sun, others from deep space, others from sources unknown. They zoom invisibly through space in numbers hard to comprehend. The current estimate is 10⁸⁹, meaning there are one billion neutrinos for every atom in the universe.

"Neutrinos are truly ghostlike particles," stated Fuller. "It's hard to stop them. It's hard to catch them. How do we even know they exist or where they are?"

We know they exist because neutrinos can be scattered and absorbed through the weak interaction—so-called because it is about 20 orders of magnitude weaker than electromagnetism. This weak interaction does allow for their detection, but makes it almost impossible to do so without going to extraordinary measures.

The weak interaction is part of their allure: it allows them to travel across the cosmos; they can escape the dense environments surrounding collapsing stars and black holes; and they're coursing through your body at a rate of 100 billion per square centimeter per second!

"Because the interaction is so weak, neutrinos can get through almost anything," said Fuller. "Light-years of lead—depending on their energy, they would go right through it."

Why do neutrino research at the South Pole?

Back to those extraordinary measures: In the 1930s, Wolfgang Pauli hypothesized the existence of neutrinos (he called them "neutrons" at the time) to explain invisible energy loss in certain kinds of radiation. At the time, he stated, "I have done a terrible thing, I have postulated a particle that cannot be detected."

And for a while he was right. But in 1956, Clyde Cowan and Frederick Reines were finally able to detect neutrinos through an experiment called "Project Poltergeist."

In the 1990s, Halzen realized he could construct a huge detector at the South Pole where ice deep in the Antarctic could be free of contaminants and bubbles that would interfere with detection. He and his team drilled holes in a cubic kilometer (0.24 cubic miles) of ice at the South Pole and placed strings of photomultiplier tubes in these holes, creating a network capable of detecting the blue light of Cherenkov radiation—a telltale visible sign of high-energy cosmic neutrinos.

IceCube is designed to detect ultra-high-energy neutrinos from deep space. These neutrinos are much rarer than the ones that come from the sun. In fact, only a few dozen are detected each year. When a rare neutrino hits the ice, it creates a fast-moving electron or muon. The muon generates a cone of Cerenkov light, allowing scientists to trace the direction the neutrino came from and estimate its energy. To Fuller, these rare neutrinos are the most tantalizing.

"Some ultra-high-energy neutrinos detected at IceCube have roughly 10 billion times the energy of a typical neutrino from the sun. One detected by a similar experiment has an energy 200 times higher than that. We don't really know where that neutrino came from," Fuller stated. "The fact that it got here at all may be saying something profound about the universe. In that sense, Halzen really is at the frontier of experimental fundamental particle physics."

UC San Diego, too, is on the quest to uncover the secret lives of neutrinos.

The Simons Observatory, built high in Chile's Atacama Desert, is collecting data on the cosmic microwave background, otherwise known as the universe's oldest light. UC San Diego was a founding institution of the observatory, and distinguished professor of physics Brian Keating is the principal investigator.

It turns out the observatory can also serve to probe the universe's oldest neutrinos and may provide the most data on deep-space neutrinos to date. Fuller says anticipated data from the observatory can help us understand the Big Bang and the early stages of the universe, among other cosmic mysteries.

"Halzen's work, and the work of those he inspired with his vision, was foundational in giving humanity a new window on the universe and on the mysterious and notoriously slippery neutrino," he stated.