Probability distributions for the value of the largest squared neutrino mass separating from NOvA data when measurements of antineutrinos in nuclear reactors are included. Normal mass order (blue) is preferred to reverse order (red) by a factor of 7:1. Credit: NOvA Collaboration
The NOvA international collaboration presented new results at the Neutrino 2024 conference in Milan, Italy, on June 17. The collaboration has doubled its neutrino data since their previous release four years ago, including adding a new sample of low-energy electron neutrinos.
The new results are consistent with previous NOvA results, but with improved precision. The data favor the “normal” ordering of neutrino masses more than ever before, but there remains ambiguity around the neutrino’s oscillation properties.
The latest NOvA data provide a very precise measure of the largest separation between squared neutrino masses and slightly favor the normal mass order. This precision on the mass separation means that, when coupled with data from other experiments in nuclear reactors, the data favor the normal order with a probability of nearly 7:1.
This suggests that neutrinos adhere to normal order, but physicists have not reached the high threshold of certainty required to declare 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 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 type as they move, a phenomenon known as neutrino oscillation.
NOvA aims to learn more about the order of neutrino masses. Physicists know that there are three types of neutrinos with different masses, but they do not know their absolute mass, nor which one is the heaviest.
Theoretical models predict two possible mass orders, normal and reversed. In the normal order, there are two light neutrinos and one heavier neutrino; in the reversed order, there is one light neutrino and two heavier ones.
“Getting more information from reactor experiments improves our understanding of mass order and brings us closer to exciting territory,” said Erika Catano-Mur, a postdoctoral research associate at William & Mary and co-organizer of the analysis. “We’re almost there to answer one of the 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 distinguish between two effects on the oscillation: mass ordering and a property called charge parity violation.
The collaboration observed a moderate amount of oscillation that could be explained in either mass order scenario with different levels of CP violation, so they cannot distinguish mass order and violation of the CP. However, physicists have been able to rule out specific combinations of these two properties.
An electron neutrino scattering event from the latest NOvA data. The brighter the yellow pixels, the greater the energy deposited. Physicists know it is a neutrino because it occurs at the same time as the beam pulse, “points” toward Fermilab, and occurs far from the edges of the detector, meaning the element at the origin of the activity must have passed through a large amount of material without leaving a trace. The electron in the final state initially resembles a track, but then expands into an electromagnetic cascade. Credit: NOvA Collaboration
“It really takes more than one measurement for us to learn everything we need to know,” said Jeremy Wolcott, a postdoctoral researcher at Tufts University, one of NOvA’s analysis coordinators and a speaker at the conference.
“NOvA is an important player in this field because there are unique aspects to different experiments that attempt to measure the same parameters,” Wolcott said. “We’re starting to see a picture emerge, but it’s blurry. It’s really important to have different measures that all work together. »
The NOvA experiment began collecting data in 2014 and will continue until early 2027, during which time the collaboration hopes to double its antineutrino data set. They are also continuing to implement analysis improvements to maximize the experiment’s sensitivity.
Their efforts also pave the way for future experiments that will seek to further contribute to solving the mysteries surrounding the properties of neutrinos.
“We want to make the most of the data,” Catano-Mur said. “What we learn, not only from the results themselves, but also from what we learn about the analysis methods, will be useful for the next generation of experiments that are currently under construction.”
Yet 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, continue to collect valuable data and produce physics insights,” said Zoya Vallari, a postdoctoral researcher at CalTech and co-organizer of the analysis. “They represent our best chance of making a discovery at this time. »
The NOvA collaboration consists of more than 200 scientists from 50 institutions in eight countries. With the additional data and improved analysis, NOvA will enable physicists to better understand the identity-changing behavior of neutrinos.
Provided by Fermi National Accelerator Laboratory
Quote: New NOvA results add to neutrino mystery (June 28, 2024) retrieved June 28, 2024 from https://phys.org/news/2024-06-nova-results-mystery-neutrinos.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without written permission. The content is provided for information purposes only.