The new NOvA results add to the neutrino mystery

The international NOvA collaboration presented new results at the Neutrino 2024 conference in Milan, Italy, on June 17. The collaboration doubled their neutrino data since their previous release four years ago, including the addition of a new low-energy sample of electron neutrinos.

The new results are consistent with the previous NOvA results, but with improved accuracy. The data favor the “normal” order of neutrino masses more strongly than before, but uncertainty remains about the properties of the neutrino oscillation.

The latest NOvA data provide a very precise measurement of the largest separation between squared neutrino masses and slightly favor the normal mass ordering. This precision in mass separation means that, when combined with data from other experiments performed on nuclear reactors, the data favors normal ordering by almost 7:1 odds.

This suggests that the neutrinos adhere to the normal ordering, but physicists have not met the high confidence threshold required to declare a discovery.

NOvA, short for NuMI Off-axis νe Appearance, is an experiment managed by the US Department of Energy’s Fermi National Accelerator Laboratory, located outside of Chicago.

Fermilab sends a neutrino beam 500 miles north to a 14,000-ton detector in Ash River, Minnesota. By measuring neutrinos and their antimatter partners, antineutrinos, in both places, physicists can study how these particles change their type as they travel, a phenomenon known as neutrino oscillation.

NOvA aims to learn more about the ordering of neutrino masses. Physicists know that there are three types of neutrinos with different masses, but they don’t know the absolute mass or which one is heaviest.

Theoretical models predict two possible mass orderings, normal or inverted. In the normal order, there are two light neutrinos and one heavier neutrino; inverted, there is one light neutrino and two heavier ones.

“Getting additional information from reactor experiments increases our knowledge of mass ordering and brings us closer to exciting territory,” said Erika Catano-Mur, a postdoctoral research associate at William & Mary and co-convener of the analysis. “We almost have an answer to one of those big questions we have in neutrino physics. But we’re not there yet.”

The resolution of the neutrino oscillation remains unclear in the new results. Physicists currently do not have enough data to separate two effects in the oscillations: mass ordering and a feature called charge parity violation.

The collaboration observed a moderate amount of swing that could be explained in each mass ordering scenario by different amounts of CP violation, so they cannot separate mass ordering and CP violation. However, physicists were able to rule out specific combinations of the two properties.

“It really takes more than one measurement for us to learn everything we need to know,” said Jeremy Wolcott, a postdoctoral fellow at Tufts University, one of NOvA’s analysis coordinators and a speaker at the conference.

“NOvA is an important player in this because there are unique aspects to all the different experiments that are trying to measure the same parameters,” Wolcott said. “We’re starting to see a unified picture, but it’s blurry. Having different measurements that all work together is really important.”

The NOvA experiment began taking data in 2014 and will continue until early 2027, during which time the collaboration hopes to double their antineutrino data. They also continue to implement assay improvements to maximize the sensitivity of the experiment.

Their efforts are also paving the way for future experiments that will try to contribute even more to solving the mysteries surrounding the properties of neutrinos.

“We want to get as much out of the data as we can,” Catano-Mur said. “What we learn—not just from the results themselves, but in the process, what we’re learning about analysis methods—will be useful for the next generation of experiments now under construction.”

However, NOvA has the potential to discover more about the elusive neutrino. “This result is an important reminder that the current generation of experiments, including NOvA, continues to collect valuable data and produce insights into physics,” said Zoya Vallari, postdoctoral researcher at CalTech and co-convener of the analysis. “They are our best discovery at the moment.”

The NOvA collaboration consists of more than 200 scientists from 50 institutions in eight countries. With the additional data and further improvements to the analysis, NOvA will bring physicists closer to understanding the identity-changing behavior of the neutrino.