How neutrinos may hold the keys to why we exist

A Michigan State University researcher has co-led a joint analysis between two major neutrino experiments, bringing scientists closer to understanding the mystery of how the universe came to be.

For the first time, the T2K experiment in Japan and the NOvA experiment in the United States have combined data from across the globe to tighten measurements of neutrino properties — the tiny, ghostlike particles that stream through the universe and rarely interact with other matter.

Together, their analysis, published in Nature, provides some of the most precise measurements of how neutrinos change types as they travel. This work lays the foundation for future experiments that could deepen understanding of how the universe evolved — or could break existing theories altogether. The material is based on work supported by the U.S. Department of Energy.

Kendall Mahn, an MSU physics and astronomy professor, helped coordinate the joint analysis and is also a co-spokesperson for T2K. Combining the two experiments’ efforts enabled the teams to achieve results far beyond what either could have done alone.

“This was a big victory for our field,” said Mahn. “This shows that we can do these tests, we can look into neutrinos in more detail and we can succeed in working together.”

When the universe began, physicists expected there would have been equal amounts of matter and antimatter. But if that were so, the matter and antimatter would have canceled each other out, resulting in total annihilation.

And yet, here we are. Somehow, matter won out over antimatter — but we still don’t know how or why.

Physicists suspect the answer may lie in the mysterious behavior of abundant yet elusive particles called neutrinos. Specifically, learning more about a phenomenon called “neutrino oscillation,” in which neutrinos change types — or flavors — as they travel, could bring us closer to an answer.

“Neutrinos are not well understood,” said MSU postdoctoral associate Joseph Walsh, who worked on the project. “Their very small masses mean they don't interact very often. Hundreds of trillions of neutrinos from the sun pass through your body every second, but they almost all pass straight through. We need to produce intense sources or use very large detectors to give them enough opportunity to interact for us to see them and study them.”

T2K and NOvA are both long-baseline experiments. They each shoot an intense beam of neutrinos that passes through a near detector close to the neutrino source and a far detector hundreds of miles away. Both experiments compare data recorded in each detector to learn about neutrinos’ behavior and properties.

Since the experiments have similar science goals but different baselines and neutrino energies, physicists can learn more by combining their data.

“By making a joint analysis, you can get a more precise measurement than each experiment can produce alone,” NOvA collaborator Liudmila Kolupaeva said. “As a rule, experiments in high-energy physics have different designs, even if they share the same science goal. Joint analyses allow us to use complementary features of these designs.”

The mystery of neutrino mass ordering centers on which neutrino is the lightest. But it isn’t as simple as placing particles on a scale. Neutrinos have tiny masses that are made up of combinations of mass states. There are three neutrino mass states, but, confusingly, they don’t map directly to the three neutrino flavors. In fact, each flavor is made of a mix of the three mass states and each mass state has a different probability of acting like each flavor of neutrino.

There are two possible mass orderings, called normal and inverted. Under the normal ordering, two of the mass states are relatively light and one is heavy, while the inverted ordering has two heavier mass states and one light state.

In the normal ordering, there is an enhanced probability that muon neutrinos will oscillate to electron neutrinos but a lower probability that muon antineutrinos will oscillate to electron antineutrinos. In the inverted ordering, the opposite happens. However, an asymmetry in the neutrinos’ and antineutrinos’ oscillations could also be explained if neutrinos violate charge-parity symmetry — in other words, if neutrinos don’t behave the same as their antimatter counterparts.

The combined results of NOvA and T2K do not favor either mass ordering. If the neutrino mass ordering is found to be normal, NOvA’s and T2K’s results are less clear on CP symmetry, requiring additional data to clarify. However, if future results show the neutrino mass ordering is inverted, the results published in Nature provide evidence that neutrinos violate CP symmetry. If there were no CP symmetry violation, then physicists would lose their best remaining explanation for why the universe is dominated by matter instead of antimatter.

These first joint results do not definitively solve any mysteries of neutrinos, but they do add to physicists’ knowledge about the particles. They also validate the impressive collaborative effort between two competing — yet complementary — experiments.

The NOvA collaboration consists of more than 250 scientists and engineers from 49 institutions in eight countries. The T2K collaboration has more than 560 members from 75 institutions in 15 countries. The two collaborations began active work on this joint analysis in 2019. It combines eight years of data from NOvA, which began collecting in 2014 and a decade of data from T2K, which started in 2010. Both experiments continue to collect data and efforts are already underway to update the joint analysis.

“These results are an outcome of cooperation and mutual understanding between two unique collaborations — both involving many experts in neutrino physics, detection technologies and analysis techniques, working in very different environments and using different methods and tools,” T2K collaborator Tomáš Nosek said.

This story originally appeared on the College of Natural Science website.