Researchers in US and Japan offer insight into ghostly neutrinos

By Will Dunham

WASHINGTON (Reuters) -Neutrinos are tiny particles that can pass through everything, rarely interacting with matter. They are the universe's most abundant particles, and trillions of them zip through our bodies every second without us noticing. Yet scientists are still struggling to understand them.

A new study that combines results from two major neutrino experiments in Japan and the United States is now offering some of the best information to date about these ghostly particles.

Neutrinos, forged 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 is providing insight into the difference in mass between neutrino types, a key unanswered question.

Neutrinos are elementary particles, meaning they are not built of anything smaller, making them one of the fundamental building blocks of the cosmos. Unlike some other particles such as protons and electrons, neutrinos lack any electric charge.

So why is it important to understand neutrinos? They might be the key to unlocking certain mysteries about the universe such as the origin of matter and its prevalence in the cosmos over its counterpart antimatter, the nature of dark matter and dark energy and the inner workings of supernovas.

The NOvA experiment sends an underground beam of neutrinos 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 beam of neutrinos about 185 miles (295 km) through the Earth's crust from its source in the Japanese seaside town of Tokai to a detector at the city of Kamioka.

Both experiments are exploring neutrino oscillation but use different neutrino energies, different distances and differently designed detectors. By combining the findings from nearly a decade of NOvA and T2K observations, the researchers made strides in the understanding of neutrinos, presented in a study published on Wednesday in the journal Nature.

"On the face of it, there were questions about whether or not the T2K and NOvA results were compatible. We learned they are very compatible," said Michigan State University physicist Kendall Mahn, co-spokesperson for the T2K research team.

Scientists do not know the mass of the three types of neutrinos or even which is the lightest, an issue scientists call "neutrino mass ordering" that has big implications for physics.

"While we will have to wait a little longer to know which neutrino is the lightest, this study measured the tiny mass gap between two of the three neutrinos with an unprecedented accuracy - less than 2% uncertainty - making it one of the most precise measurements of this parameter ever achieved," Ohio State University physicist and NOvA scientist Zoya Vallari said.

The two experiments also are delving into whether neutrinos and their counterpart particles, called antineutrinos, change from one type to another differently from each other.

"That question is especially important because it may help explain one of the biggest mysteries in physics: why the universe is made mostly of matter instead of antimatter. At the Big Bang, matter and antimatter should have existed in equal amounts and destroyed each other. But somehow, matter won, and we're here because of it," Vallari said.

Answering fundamental questions about the universe requires extremely high precision and statistical confidence, Vallari said, and another generation of large neutrino experiments is on the horizon.

The Fermilab-led DUNE experiment is under construction in Illinois and South Dakota. Hyper-Kamiokande is under construction in Japan's Gifu Prefecture. Other efforts already underway include a project in China called JUNO and telescopes that capture neutrinos coming from space such as KM3NeT and IceCube.

"Neutrinos have unique properties, and we are still learning a lot about them," Mahn said.

(Reporting by Will Dunham; Editing by Daniel Wallis)