Probability distributions for the value of the larger neutrino mass quadratic splitting of NOvA data when including antineutrino measurements at nuclear reactors. The normal (blue) mass ordering is preferred over the inverse (red) by a factor of 7:1. Credit: NOvA Collaboration
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 adding a new low-energy sample of electron neutrinos.
The new results are consistent with previous NOvA results, but with improved accuracy. The data argue more strongly for the ‘normal’ ordering of the neutrino masses than before, but ambiguity remains about the neutrino’s oscillatory properties.
The latest NOvA data provide a very precise measure of the larger splitting between the squared neutrino masses, and slightly favor the normal mass ordering. That precision on the mass splitting means that, when coupled with data from other experiments performed at nuclear reactors, the data favor the normal ordering by nearly 7:1.
This suggests that neutrinos obey the normal order, but that physicists have not yet reached the high threshold of certainty needed to explain a discovery.
NOvA, short for NuMI Off-axis νe Appearance, is an experiment operated by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, located outside Chicago.
Fermilab sends a beam of neutrinos 500 miles (800 kilometers) north to a 14,000-ton detector in Ash River, Minnesota. By measuring the neutrinos and their antimatter partners, antineutrinos, at both locations, physicists can study how these particles change type as they travel, a phenomenon known as neutrino oscillation.
NOvA wants to learn more about the ordering of neutrino masses. Physicists know that there are three types of neutrinos with different masses, but they do not know what the absolute mass is, nor which is heaviest.
Theoretical models predict two possible mass orders: normal or inverted. In the normal order there are two light neutrinos and one heavier neutrino; conversely, there is one light neutrino and two heavier ones.
“Getting additional information from reactor experiments increases our understanding of the ordering of masses and brings us close to exciting territory,” said Erika Catano-Mur, a postdoctoral research associate at William & Mary and co-creator of the analysis. “We’re close to answering one of those big questions we have in neutrino physics. But we’re not there yet.”
The solution to neutrino oscillation remains ambiguous in the new results. Physicists currently do not have enough data to disentangle two effects on the oscillation: mass ordering and a property called charge parity violation.
The collaboration found a moderate amount of oscillation that could be explained in both mass ordering scenarios with different amounts of CP violation, so they could not distinguish between mass ordering and CP violation. However, the physicists were able to rule out specific combinations of the two properties.
An electron neutrino scattering event from the latest NOvA dataset. The brighter the yellow pixels, the more energy was deposited. Physicists know this is a neutrino because it occurs in time with the beam pulse, points back to Fermilab, and occurs far from the edges of the detector, meaning that whatever initiated the activity had to travel through a lot of matter without leaving a trace. trace. The final-state electron is initially a trace, but then evolves into an electromagnetic cascade. Credit: NOvA collaboration
“It takes more than one measurement to know everything we need to know,” said Jeremy Wolcott, a postdoctoral researcher at Tufts University, one of NOvA’s analysis coordinators and the conference speaker.
“NOvA is a key 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 picture emerge, but it’s unclear. It’s really important to have different measurements that are all working together.”
The NOvA experiment began collecting data in 2014 and will continue until early 2027, during which time the collaboration hopes to double their antineutrino dataset. They also continue to implement analysis improvements to maximize the sensitivity of the experiment.
Their efforts also pave 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 the most out of the data,” says Catano-Mur. “What we learn – not only from the results themselves, but also from the analytical methods along the way – will be useful for the next generation of experiments 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 yield physics insights,” said Zoya Vallari, postdoctoral researcher at CalTech and co-founder of the analysis. “They are our best chance for a discovery at this point.”
The NOvA collaboration consists of more than 200 scientists from 50 institutes in eight countries. With the additional data and further analysis improvements, NOvA brings physicists closer to understanding the identity-changing behavior of neutrinos.
Provided by Fermi National Accelerator Laboratory
Quote: New NOvA results add to neutrino mystery (2024, June 28) Retrieved June 28, 2024, from https://phys.org/news/2024-06-nova-results-mystery-neutrinos.html
This document is protected by copyright. Except for fair dealing for the purpose of private study or research, no part may be reproduced without written permission. The contents are provided for information purposes only.