The Innovation Platform investigates how the NOvA experiment at Fermilab is working to determine the role of mysterious neutrino particles.
Neutrinos are tiny subatomic particles and are the most abundant matter particle in the Universe. Despite this, however, they have very little interaction with other matter and are often termed ‘ghost particles’ as a result. Currently, little is known about these mysterious neutrino particles and the role they play in the evolution of the cosmos.
The NOvA (NuMI Off-axis ve Appearance) experiment, managed by the Fermi National Accelerator Laboratory near Chicago, works to shed light on these elusive particles. Fermilab sends a beam of neutrinos 500 miles north to a 14,000-ton detector in Ash River, Minnesota. By measuring the neutrinos and their antimatter partners, antineutrinos, in both locations, physicists can study how these particles change their type as they travel – a phenomenon known as neutrino oscillation.
NOvA aims to discover more about the ordering of neutrino masses. Physicists know that there are three types of neutrinos with different masses, but they do not know the absolute mass, nor which is heaviest. Theoretical models predict two possible mass orderings: Normal or inverted. In the normal ordering, there are two light neutrinos and one heavier neutrino. In inverted, there is one light neutrino and two heavier ones.
The experiment is a collaboration of over 200 scientists, engineers, graduate students, and undergraduate students from 50 different institutions in eight different countries including the US, UK, Russia, India, Turkey, Colombia, Brazil, and the Czech Republic.
To find out more about the NOvA experiment’s goals and recent progress, Editor Maddie Hall spoke to NOvA Co-spokespersons Patricia Vahle, Professor of Physics at William & Mary, and Alex Himmel, Scientist at Fermilab.
Can you describe the NOvA experiment and its objectives?
NOvA is a long baseline oscillation experiment focused on studying neutrino particles produced at Fermilab.
Initially, the neutrinos are generated in an accelerator before the beam is measured at two distinct locations. The first is the near detector located at Fermilab, which acts as a control and enables analysis of the beam before it undergoes any changes. The neutrinos then travel through the Earth, covering a distance of 810km, and reach our much larger, far detector located in northern Minnesota that detects the more diffuse beam. The measurements from the two detectors are compared, aiming to identify changes in the energy and flavour distribution of the neutrinos. Specifically, the experiment observes any alterations in the number of neutrinos at different energy levels and the distribution of different types of neutrinos – electron, muon, or tau neutrinos. These changes are evidence of neutrino oscillations.
Precise measurements of how both neutrinos and antineutrinos oscillate tell us the differences in the masses squared of the neutrinos and how much the neutrinos like to mix. We also hope to learn the ordering of the masses of the neutrinos and whether neutrinos and antineutrinos oscillate the same way. If they don’t, then neutrinos might be violating charge-parity (CP) symmetry.
Adjusting the current running through our focusing elements enables us to conduct these measurements using both neutrinos and antineutrinos. The neutrinos and antineutrinos oscillations can then be compared to take the required measurements.
What is the significance of neutrino oscillations in particle physics research?
Neutrinos are fundamental particles, meaning they cannot be broken down into smaller components. They are also the most abundant matter particle in the Universe, but knowledge of their properties is limited, including their mass, which is one of the most significant things to learn about a particle.
Though neutrinos must have mass to undergo oscillations, the exact mass is so small that it has not yet been measured. We know that the difference in mass between two of the neutrinos is small, while the other difference in mass is relatively large, but we don’t know the arrangement of these differences. It could be that we have two light neutrinos and one heavier neutrino, which is the normal mass ordering, or it could be flipped around, with one light neutrino and two heavier neutrinos – the inverted mass ordering. Understanding the mass ordering is an important element of understanding the process by which neutrinos get their mass, and why their masses are so small.
Beyond learning about how the neutrino behaves, we also want to learn more about the weak force. Most other particles have multiple types of interactions, meaning the weak force is often difficult to study, as it is often overshadowed. Uniquely, neutrinos exclusively interact through the weak force. Though this means interactions are rare, they also provide an excellent opportunity to understand the weak force in isolation.
For quite some time now, particle physics has focused on trying to understand the limitations of the Standard Model of particle physics. The Standard Model has been very successful in describing all known particles and forces and has made highly precise predictions. However, it is widely accepted that this model is not complete and cannot account for very high-energy phenomena.
The overarching goal of particle physics is to uncover what lies beyond the Standard Model and to understand the deeper level of fundamental particles and forces. Experimental evidence is required to determine which of the numerous theories is accurate.
Neutrino oscillations represent compelling laboratory evidence of physics not explained by the Standard Model. While this evidence does not provide a clear blueprint for a new model, it is crucial to study because it offers the strongest indication so far that there are aspects of physics not accounted for by the standard model.
The NOvA team presented recent key findings at the Neutrino 2024 conference in Italy in June. Can you detail the importance of these results?
Particle physicists are investigating whether neutrino particles violate a fundamental concept known as charge parity (CP) symmetry. This symmetry involves changing all particles into their corresponding antiparticles and observing if the physical laws remain the same when viewed in a mirror.
Understanding this is crucial because, in the early Universe, it is thought that equal amounts of matter and antimatter were produced. Nowadays, when we look out into the Universe, we see a whole lot of matter but not much anti-matter. This creates two important questions. First, where did all the expected antimatter go? Secondly, why does any matter remain to create galaxies, stars, planets, and life as we know it? Answering these questions may depend on studying the violation of charge parity symmetry in neutrinos.
Unfortunately, our experiments have shown that mass ordering and violation of charge parity symmetry have similar effects. The latest results presented at the Neutrino 2024 conference suggest that it is currently not possible to differentiate between the impacts of mass ordering and CP violation. If mass ordering and CP violations move in the same direction, it is clear that they move together. However, if they move in countervailing directions, it is impossible to tell which one is moving in which way. Consequently, we could be in a situation with normal mass ordering and very little CP violation or even CP symmetry. On the other hand, it could be a case of an inverted mass ordering with significant CP violation.
The NOvA experiment is not the only one trying to answer the questions of mass ordering and CP violation. Our colleagues in Japan, who are working on the T2K experiment, are inclining toward a scenario where the two effects move together, indicating normal ordering and a significant amount of CP violation. If the NOvA results had aligned with theirs, we would have been more confident in our understanding of these parameters. However, as it stands, with only two measurements and differences between them, these questions remain unanswered and may continue to be a mystery in the next generation of experiments.
Neutrino 2024 featured another significant result from NOvA – the most precise measurement of the larger mass splittings from a single experiment. This measurement’s precision is now at the 1.5% level. Achieving the ability to measure these parameters with accuracy below 2% marks a significant achievement in the field.
How does the collaboration approach precision in NOvA?
The experiment has doubled the number of neutrinos in our dataset, significantly increasing the statistical precision of our measurements. The collaboration collects data on both neutrinos and antineutrinos, and in 2024, we presented twice the amount of neutrino data. Attention is now turning to doubling our antineutrino dataset, which will give us additional power to interpret the findings.
More data will enable more precise measurements. Due to the small quantity of neutrinos initially present and the low probability of muon neutrinos transforming into electron neutrinos, the numbers involved are very small.
However, neutrino oscillations predict a specific pattern of behaviour at different energies and baselines, so combining measurements with different baselines is a powerful handle for confirming the oscillation model. Already, results from studies of reactor neutrinos are integrated into NOvA analysis. There are or will be new accelerator experiments at shorter baselines (T2K, Hyper-K) and a longer baseline (DUNE), and seeing consistent oscillations across all these experiments would be powerful confirmation of the oscillation model. If, on the other hand, the experiments disagree at different baselines, it may point us towards something new beyond the oscillation model.
NOvA is a large international collaboration. Why is collaboration so important in this research?
Collaboration is essential due to the scale of our detectors, and the construction of such massive infrastructure necessitates extensive teamwork. Projects such as DUNE require funding and contributions from multiple countries to build the necessary devices for scientific research. It is also crucial to have a diverse group of collaborating scientists. This diversity enables different perspectives and solutions to problems, leading to enhanced scientific outcomes from our experiments.
What challenges are faced when conducting an experiment like this?
The fundamental challenge in the study of neutrinos is the rarity of their interactions. At our far detector, we are lucky to detect a neutrino per day. Each neutrino is extremely precious, meaning it is crucial to maximise detection capability. This is primarily achieved in three ways:
- Increasing the power of the beam: Producing as many neutrinos as possible is essential for data collection. In June, Fermilab set a new record with its powerful beam, producing a one-megawatt beam – an impressive milestone for neutrino beams.
- Staying up and running: Maintaining a constant detector operation is vital, with round-the-clock staffing and a responsive on-call crew contributing to the experiment’s uptime of 98%. Our detectors can also detect supernovae, so we aim to keep them running continuously, even in the absence of the neutrino beam, in case of a supernova occurrence.
- Eliminating background interferences: Unlike the typical external sources, our primary background interference comes from other neutrinos in the beam that lack the necessary oscillation information. To address this challenge, we have implemented Convolutional Neural Networks, a cutting-edge computer vision technique, enabling us to maximise data analysis.
The two detectors must also be managed differently due to their locations and requirements. The far detector is larger and is located in a remote area. Thus, it has dedicated, professional staff stationed near the detector to take care of it.
On the other hand, the near detector is smaller and is located at Fermilab. Graduate students and postdocs working on the experiment typically handle the hardware work for this detector. For instance, the recent upgrades to the near detector, involving the replacement of most of the electronics, were mainly carried out by the experiment’s students. The advantage of the smaller size of the new detector is that it doesn’t need dedicated staff for maintenance, and there are plenty of people with the required expertise available to assist.
What are the future steps for the NOvA experiment regarding data collection and analysis?
The next step is to double our antineutrino dataset, which is already underway. This is expected to continue until the end of NOvA, which is the end of the calendar year 2026. Though the experiment is mature and does not expect significant performance increases, we aim to extract every bit of sensitivity improvement possible. A number of improvements have been implemented concerning the detection of activity associated with a neutrino interacting in our detector, as well as in our models of how those neutrinos interact in our detector. Additionally, we are exploring more extensive use of machine learning techniques to enhance the precision of our measurements.
We are currently working on several physics programmes. In addition to studying supernova neutrinos, we are also measuring the cross-section of neutrino interactions in our near detector. This data is not only valuable for our experiment but also for future experiments. Having gathered a wealth of data on neutrino interactions, cross-sections from various kinematic variables can be extracted.
Additionally, we are searching for more exotic phenomena, such as detecting dark matter produced in our beam and decaying before reaching our detector. A recent publication in this area concerned a search for non-standard interactions. The oscillation model may not be as simple as once thought, and non-standard interactions may offer an alternative version. While measurements were conducted to assess the alignment of our data with the model, strong evidence was not found.
The collaboration remains enthusiastic about the experiment. The 72 PhD and masters’ students currently dedicated to their work on this experiment reflect the high level of interest that persists in studying neutrino oscillations. With exciting results anticipated in the coming years, be sure to stay updated on our progress.
Please note, this article will also appear in the 19th edition of our quarterly publication.
