Two leading experiments reveal combined insights into why matter exists

Inside the Super-Kamiokande detector. (Image courtesy of Super-Kamiokande collaboration)

The following story originally appeared on the website for W&M's School of Computing, Data Sciences & Physics. - Ed.

At the moment of the Big Bang, matter and antimatter were created in equal amounts - or at least that's what the standard laws of physics predict. If that were the whole story, however, matter and antimatter would have destroyed each other completely, leaving behind only energy.

Yet here we are.

The explanation - a small excess of matter survived.

Could neutrinos - ghost-like particles that barely interact with anything - help explain why the universe exists in its current form?

Tricia Vahle, a professor of physics at William & Mary, recalls first being introduced to the concept of neutrinos when she was in high school.

"At the time they called them anomalies," said Vahle. "Neutrinos are produced in the sun through fusion reactions, and scientists were trying to measure how many of these neutrinos would make it here on earth. They found far too few of them to explain how bright the sun was."

How did these neutrinos go missing? This has been the question particle physicists have been asking for decades.

Two separate experiments - the NOvA collaboration at Fermilab in Illinois and the T2K experiment in Japan - are working to answer this question, which may help reveal how our universe formed.

For the first time, the two international collaborations combined their results, published in October in the journal Nature.

International collaborations

There are three neutrino flavors, and scientists discovered they can change their identity as they travel, a phenomenon called neutrino oscillation. A neutrino begins as one flavor, but as it travels thousands of miles through the earth, the flavor can change.

Both NOvA and T2K aim to learn about the neutrinos' behavior and properties by shooting an intense beam of neutrinos to a detector hundreds of miles away.

"Most neutrinos stream right into space and aren't detected, but with enough neutrinos, and a large enough detector, sometimes we get lucky," said Vahle, who is the co-spokesperson for the NOvA experiment.

The NOvA collaboration includes more than 250 scientists and engineers from 49 institutions across eight countries. An intense beam of neutrinos are fired from Fermilab straight through the crust of the earth and measured at a 14,000-ton detector located in Ash River, Minnesota, with the particles taking less than three milliseconds to make the 810-kilometer trip.

The T2K collaboration includes more than 560 members from 75 institutions across 15 countries. For T2K, a neutrino beam travels 295 kilometers from Tokai to Kamioka, where the Super-Kamiokande neutrino detector is located.

Vahle said combining the results helps them understand the strengths and challenges of the two experiments.

"We measure neutrino oscillations in slightly different ways: different detectors, different distances the neutrinos travel, different energies," said Vahle. "When we combine our results, we can make a stronger statement about neutrinos than we could separately."

The result provides the most precise measurement to date of the differences in the masses of two neutrinos, but there remain two possible scenarios.

If neutrinos follow the inverted mass ordering, with one light and two heavier neutrinos, then neutrinos break a fundamental rule called charge-parity symmetry, which could help explain why matter won out over antimatter. On the other hand, if neutrinos follow a normal mass ordering with two light and one heavy neutrino, then it remains unclear whether neutrinos could help explain the matter vs. antimatter asymmetry.

What comes next

This first joint analysis of the two experiments included six years of data from NOvA and eight years of data from T2K.

Vahle said she expects the collaboration across the two experiments to continue as they collect new data and forthcoming neutrino experiments come online.

"Nature is revealing that our current models of nature are lacking," stated Vahle. "We are learning something new, something that we got wrong, and that will lead us to think about the problem in different ways and come up with new solutions."

Eva Kalajian, W&M's School of Computing, Data Sciences & Physics

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