By Will Dunham
WASHINGTON (Reuters) – Neutrinos are tiny particles that can pass through anything and rarely interact with matter. They are the most abundant particles in the universe, and trillions of them pass through our bodies every second without us noticing. But scientists are still trying to understand them.
A new study combining results from two major neutrino experiments in Japan and the US now offers the best information yet on these ghostly particles.
Neutrinos produced in places like the sun’s core and exploding stars come in three types, or “flavors,” and can change from one to another—called oscillation—as they travel. The new study provides insight into the mass difference between the types of neutrinos, a key unanswered question.
Neutrinos are elementary particles, meaning they are not made of anything smaller, making them one of the fundamental elements of the cosmos. Unlike some other particles, such as protons and electrons, neutrinos lack an electrical charge.
So why is it important to understand neutrinos? They may hold the key to certain mysteries of the Universe, such as the origin and distribution of matter in space versus antimatter, the nature of dark matter and dark energy, and the inner workings of supernovae.
The NOvA experiment sends an underground neutrino beam about 500 miles (810 km) from its source at the U.S. Department of Energy’s Fermi National Accelerator Laboratory near Chicago to a detector in Ash River, Minnesota. The T2K experiment sends a neutrino beam about 185 miles (295 km) through the Earth’s crust from its source in the Japanese seaside town of Tokai to a detector in Kamioka.
Both experiments study neutrino oscillations, but use different neutrino energies, different distances, and different detector designs. By combining findings from nearly a decade of NOvA and T2K observations, scientists have made progress in understanding neutrinos in a study published Wednesday in the journal Nature.
“At first there were questions about whether or not the T2K and NOvA results were compatible. We found that they are very compatible,” said Michigan State University physicist Kendall Mahn, a spokesperson for the T2K research group.
Scientists do not know the mass of the three types of neutrinos, or even which one is the lightest, scientists call this problem the “neutrino mass order”, which has great implications for physics.
“Although we’ll have to wait a little longer to find out which neutrino is the lightest, this study measured the small mass gap between two of the three neutrinos with unprecedented precision — less than 2% uncertainty — making it one of the most precise measurements ever made of this parameter,” said Ohio State University physicist and NOvA scientist Zoya Vallari.
The two experiments also investigate whether neutrinos and their counterpart particles, called antineutrinos, change from one type to another in different ways.
“That question is particularly important because it could help explain one of the biggest mysteries in physics: why the universe is made up mostly of matter and not antimatter. At the Big Bang, matter and antimatter should have existed in equal amounts and annihilated each other. But somehow matter won, and that’s why we’re here,” Vallari said.
Answering fundamental questions about the universe requires extreme precision and statistical confidence, Vallari says, and another generation of large neutrino experiments is on the horizon.
The Fermilab-led DUNE experiment is being built in Illinois and South Dakota. Hyper-Kamiokande is being built in Japan’s Gifu Prefecture. Other efforts already underway include a project in China called JUNO and telescopes that capture neutrinos from space, such as KM3NeT and IceCube.
“Neutrinos have unique properties, and we’re still learning a lot about them,” Mahn said.
(Reporting by Will Dunham; Editing by Daniel Wallis)