Probability distribution for the value of the neutrino mass squared division from the NOvA data when antineutrino measurements in nuclear reactors are included. The normal (blue) arrangement of matter is favored over the inverted (red) by a ratio of 7:1. Credit: NOvA Collaboration
The international NOvA collaboration presented the new results at the Neutrino 2024 conference in Milan, Italy on June 17. The collaboration doubled their neutrino data since their previous publication four years ago, including the addition of a new low-energy electron neutrino sample.
The new results are consistent with previous NOvA results, but with improved accuracy. The data favors the “normal” arrangement of neutrino masses more strongly than before, but ambiguity remains around the oscillatory properties of neutrinos.
The latest NOvA data provide a very precise measurement of the larger distribution between the squared neutrino masses and slightly favor the normal mass ordering. This precision of the mass distribution means that, when combined with data from other experiments conducted in nuclear reactors, the data favors the normal arrangement with almost a 7:1 probability.
This suggests that the neutrinos follow a normal arrangement, but physicists have not met the high confidence threshold needed to declare the discovery.
NOvA, short for NuMI Off-axis νe Appearance, is an experiment run by the U.S. 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, at both locations, physicists can study how these particles change their type as they travel, a phenomenon known as neutrino oscillations.
NOvA aims to learn more about the arrangement 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 weight arrangements, normal or inverted. In the normal arrangement, there are two light neutrinos and one heavier neutrino; in the inverted one there is one light neutrino and two heavier ones.
“Gaining more information from reactor experiments will expand our understanding of bulk ordering and get us close to exciting territory,” said Erika Catano-Mur, a William & Mary postdoctoral researcher and co-organizer of the analysis. “We almost have an answer to one of the big questions we have in neutrino physics. But we’re not there yet.”
The neutrino oscillation solution remains ambiguous in the new results. Physicists currently do not have enough data to separate two effects on the oscillation: the arrangement of matter and a property called charge parity violation.
The collaboration observed a modest amount of oscillation that could be explained in both bulk ordering scenarios with different amounts of CP violation, so they cannot separate bulk ordering and CP violation. However, physicists have been able to rule out specific combinations of these two properties.
Electron neutrino scattering event from the latest NOvA data set. The brighter the yellow pixels, the more energy has been stored. Physicists know it’s a neutrino because it occurs in time with the beam pulse, “points” back at Fermilab, and occurs far from the edges of the detector, meaning that whatever initiated the activity must have passed through a large amount of matter without left a trace. The final state electron is initially orbital, but then evolves into an electromagnetic cascade. Credit: NOvA Collaboration
“It really takes more than one measurement to learn everything we need to know,” said Jeremy Wolcott, a postdoctoral fellow at Tufts University, one of the coordinators of the NOvA analysis and a speaker at the conference.
“NOvA is an important player in this because all the different experiments trying to measure the same parameters have unique aspects,” Wolcott said. “We’re starting to see the picture come together, but it’s cloudy. Having different measurements that all work together is really important.”
The NOvA experiment began acquiring data in 2014 and will continue until early 2027, during which the collaboration hopes to double their antineutrino dataset. They also continue to implement assay improvements to maximize the sensitivity of the experiment.
Their efforts also pave the way for future experiments that will seek to contribute even more to solving the mysteries surrounding the properties of neutrinos.
“We want to get the most out of the data that we can,” Catano-Mur said. “What we learn—not just from the results themselves, but in the process, what we learn about analysis methods—will be useful for the next generation of experiments that are currently under construction.”
Still, NOvA has the potential to reveal 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 generate physical insights,” said Zoya Vallari, a postdoctoral fellow at CalTech and co-organizer of the analysis. “They are our best shot at discovery right now.
The NOvA collaboration consists of more than 200 scientists from 50 institutions in eight countries. With additional data and further improvements to the analysis, NOvA will bring physicists closer to understanding the identity-changing behavior of neutrinos.
Provided by Fermi National Accelerator Laboratory
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