Neutrinos have always been difficult to study because their small mass and neutral charge make them especially elusive. Scientists have made a lot of headway in the field and can now detect three flavors, or oscillation states, of neutrinos. Other flavors continue to be elusive — though that could be because they don’t even exist.
Sterile neutrinos, a flavor that has been proposed to play a role in neutrino mass generation and causing the oscillations of other neutrinos, have been hinted at in previous experiments but never detected. In a study published February 26 in Physical Review Letters, NOvA collaboration scientists did not find evidence of sterile neutrinos, but their work puts the tightest constraints on parameter space to date for where sterile neutrinos could be found.
“Neutrinos are really interesting because they can give a window into some really big questions for physics, including the question of the matter-antimatter asymmetry and why the universe exists at all,” says co-lead author Adam Lister, a postdoctoral researcher in physics professor Brian Rebel’s group at the University of Wisconsin–Madison and member of the NOvA collaboration. “It turns out you can get at these questions with neutrino oscillations, and in this paper, we tried to address the question of, ‘What if there are more types of neutrinos that we haven’t yet been able to observe?’”
Sterile neutrinos have been proposed for a couple of decades, when previous experiments showed results that could best be explained by the existence of a fourth (or greater) flavor of neutrino.
“There are a number of experiments that claim that they see something consistent with this new neutrino, and there’s a bunch of other experiments that have looked for those same neutrinos and seen nothing whatsoever,” Lister says. “It’s a very open question right now.”
To search for sterile neutrinos, the NOvA experiment produces a beam of one flavor of neutrino, muon neutrinos, at Fermilab and directs it toward two detectors: a near detector about one kilometer from the beam source, and a far detector around 800 kilometers away in Ash River, MN. Previous NOvA analyses measured only the neutral current disappearance rate, where any of the three flavors of neutrinos converts to another particle when it interacts with a detector. So in this new study, the researchers knew that if they observe an unexpected change in the total number of neutrinos, it suggests that sterile neutrinos did in fact influence the other types.
Earlier NOvA work only measured this rate at the far detector, because it was the best developed analysis strategy available at the time. However, it assumed that sterile neutrinos could not influence the other neutrinos in the first kilometer. Here, the team used updated software and improved simulations to analyze neutral current rate changes at both near and far detectors, but these results did not deviate from the expected three-flavor model.
Second, they introduced an additional sample, known as muon neutrino charged current interactions, or the disappearance specifically of muon neutrinos at both the near and far detectors. Small changes from the expected rate could also indicate the influence of sterile neutrinos, but again, they did not observe a statistically significant rate difference.
Combined with the fact that the team applied their analyses to six years of cumulative data — all the data of previous NOvA analyses plus newer data — this new study offers NOvA’s most powerful analysis of sterile neutrino physics to date. Though the study could not confirm the existence of sterile neutrinos, NOvA scientists were able to search the four different samples together simultaneously, allowing them to rule out certain parameter combinations that are physically incompatible with sterile neutrinos.
“Our results agree with the standard three-flavor oscillation model, at least with the statistical uncertainties we have,” Lister says. “What we can say right now is that NOvA sets some of the strongest limits on the existence of the sterile neutrinos.”
The NOvA experiment could not confirm the existence of the sterile neutrino. IceCube, led by UW–Madison, performs complementary searches using atmospheric and astrophysical neutrinos and has also not found any evidence for sterile neutrinos.
These results are also critical to informing the analyses of the next-gen accelerator-based neutrino detector, DUNE, which Lister and colleagues at UW–Madison are already developing.
This study was published by the NOvA collaboration, centered at Fermilab, and largely led by scientists at UW–Madison (Lister, Rebel, and former graduate student Harry Hausner) and the University of Cincinnati (postdoctoral researcher V. Hewes and physics professor Adam Aurisano).
This work was supported by the U.S. Department of Energy; the U.S. National Science Foundation; the Department of Science and Technology, India; the European Research Council; the MSMT CR, GA UK, Czech Republic; the RAS, MSHE, and RFBR, Russia; CNPq and FAPEG, Brazil; UKRI, STFC and the Royal Society, United Kingdom; and the state and University of Minnesota.
By Christopher Woolford, Physics PhD student
Over 200 faculty and students represented the University of Wisconsin–Madison at the annual American Physical Society Division of Plasma Physics (DPP) research conference in Atlanta last October. The APS DPP conference presents an excellent opportunity to learn about the newest and cutting-edge research in plasma physics.
Rogerio Jorge, an assistant professor of physics at UW–Madison, presented his work on stellarator optimization alongside several of his students. Stellarators are a type of nuclear fusion reactor that uses twisted magnetic fields to confine fusion reactions.
“The weather was amazing, and the conference is a great opportunity for collaborations,” Jorge says.
This year, the APS DPP conference benefited from attendance by a new generation of plasma physicists that has seemed to have grown in recent years.
“I had not seen so many younger plasma physicists in years at an APS DPP conference,” says Paul Terry, a professor of physics at UW–Madison, attributing the rise in attendance to the influx of private companies interested in nuclear fusion.
Eduardo Neto, a postdoc at UW–Madison working with Jorge, gave a talk on his exciting work on Stellarator optimization. This work was the result of an ongoing collaboration with Proxima Fusion and IST in Lisbon, Portugal.
“The most challenging part of this project was getting a lot of different codes to work together,” Neto says. “Startup companies have a strong interest in preventing tungsten buildup in Stellarators.”
Neto plans to continue his collaborations with Proxima Fusion and explore optimizing other parts of Stellarators such as turbulence. His advice for students interested in Stellarator optimization is to gain broad knowledge of plasma physics and nuclear fusion.
The APS DPP conference will be in November this year in Long Beach, California.
Neutrinos are weakly interacting particles that are able to travel undeflected through the cosmos. The IceCube Neutrino Observatory and the KM3NeT Astroparticle Research with Cosmics in the Abyss (ARCA) telescope (still under construction) are cubic-kilometer-scale neutrino telescopes that search for the sources of these astrophysical neutrinos in hopes of uncovering the origin of ultra-high-energy cosmic [...]
Read the full article at: https://wipac.wisc.edu/probing-the-connection-between-the-highest-energy-astrophysical-neutrinos-and-ultra-high-energy-cosmic-rays/String theorist Jakob Moritz joined the faculty as an assistant professor of physics on January 1, 2025. He joins us from CERN where he has been a postdoc for just over a year. Previously, he was a postdoc for four years at Cornell University, and before that, he earned his PhD from the University of Hamburg and DESY.
Please give a general overview of your research.
I work on string theory, a theoretical framework for quantum gravity. It is the only known approach that consistently combines quantum mechanics and Einstein’s theory of gravity. Physicists have struggled for decades to reconcile these two fundamental theories, and string theory achieves this unification. Sometimes called “the theory of everything,” string theory addresses physical phenomena at arbitrarily high energies. While the nickname may sound a bit grandiose, it highlights the theory’s incredible scope.
However, while the field equations of string theory have solutions that are relatively easy to study, these don’t resemble our universe. My research focuses on going beyond these “easy” solutions to find ones that better match the universe we observe. By doing so, I aim to uncover insights into the origins of the peculiar laws of physics governing our universe.
Something that I find particularly interesting is dimensionless constants of nature. These constants are significant because they are independent of a choice of units. For example, the ratio of the electron’s mass to the top quark’s mass is a dimensionless number — about 0.000003, which is remarkably small! There are many such constants whose values are determined experimentally, yet we lack a theoretical explanation for them.
In the early 20th century, particle physicists didn’t focus much on questions like, “Why are the constants of nature what they are, and not something else?” But with string theory, we can begin to address this. My work seeks to identify solutions of string theory in which these numbers align with experimental values. Another well-known example is the energy density of the vacuum, or dark energy. Despite being the dominant energy source in the universe today, dark energy is extraordinarily small in natural units — just 10^{-120} when compared to the natural energy scale of quantum gravity. This discrepancy, known as the cosmological constant problem, is something I find deeply intriguing. How can such a small value arise? Why isn’t it zero? Similarly, why is the Higgs mass so small? These are the kinds of profound questions I aim to explore through string theory.
What are one or two main projects you’ll have new group members work on?
One major project will involve finding the Standard Model of particle physics within string theory. This is something I am already working on, but having more hands on deck would be invaluable. The goal is to “engineer” realistic laws of particle physics — either the Standard Model or something close to it — as solutions of string theory. This work is crucial for addressing the electroweak hierarchy problem: why is the Higgs mass so unnaturally small? Currently, no one has a clear explanation for this.
Technically, this involves a lot of geometry. String theory predicts the existence of extra dimensions, which are both a blessing and a curse. They must be small enough to have remained unobservable, yet they also determine the physical laws we experience at larger scales. Much of our work will focus on understanding these geometries — particularly how certain objects, called branes, wrap around features like circles in these spaces — and calculating the resulting physical laws.
What attracted you to Madison and the university?
I really appreciate the breadth of the theory department here. String theory is a vast field, encompassing topics that range from almost pure mathematics to particle phenomenology. Because my work leans toward the phenomenological side, it intersects with many other areas of theoretical physics, including cosmology, particle physics, and applied mathematics. Being at a large place like Madison, with its diverse and talented faculty, is incredibly exciting.
Additionally, I know that Madison attracts outstanding students who are eager to work on string theory and particle physics. That’s something I’m looking forward to as well!
What is your favorite element and or elementary particle?
Neutrinos are cool because they’re almost massless. For a long time, they were thought to have zero mass, as predicted by the Standard Model of particle physics. But experiment has revealed otherwise! This discovery hints strongly at new physics at high energies.
What hobbies and interests do you have?
I love music! I play piano and guitar, and music is a big part of my life, especially since my partner is also a musician. I also enjoy sailing. While at Cornell, I spent summers sailing and participated in weekly competitive races, which were incredibly fun. I know that sailing is also a thing here — I look forward to getting back on the water!
The illustrious career of Baha Balantekin, the Eugene P. Wigner professor of physics at UW–Madison, was celebrated recently at the Neutrinos in Physics and Astrophysics Workshop through the Network for Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Physics Frontier Center.
Balantekin works at the intersection of nuclear physics, particle physics, and astrophysics. For much of his career, he has studied theoretical aspects of neutrinos originating in the Sun, supernovae, or neutron star mergers. He has helped pioneer the field known as neutrino astronomy.
“Even just a few decades ago, if you said ‘neutrino astronomy,’ most physicists would have snickered. That’s because astronomy is about observations and neutrinos are almost impossible to detect,” says John Beacom, PhD ’97, distinguished professor of physics and astronomy at the Ohio State University. “But, over time, physicists have helped to make this seemingly impossible field into something real and vibrant. The observations of astrophysical neutrinos that have been made have been essential to understanding our Sun, supernovae, and distant galaxies.”
Balantekin and George Fuller, a distinguished professor of physics at the University of California, San Diego, have helped lead the field of neutrino astrophysics through both their scientific work and their mentoring of junior scientists. To honor both scientists’ significant and ongoing contributions to the field, three of their former students organized the workshop: Beacom, a former student of Balantekin’s, and Fuller’s former students Gail McLaughlin, distinguished university professor of physics at North Carolina State University and Yong Zhong Qian, professor of physics and astronomy at the University of Minnesota. The event was held Jan 16-18 at the University of California, Berkeley.
Francis Halzen, a current colleague of Balantekin’s at UW–Madison, was one of the speakers. Other attendees included UW–Madison physics professor Pupa Gilbert and professor emerit Sue Coppersmith.
John Beacom and Pupa Gilbert contributed significantly to this story
Congrats to Prof. Dan McCammon for earning the Distinguished Career Award from The American Astronomical Society’s (AAS) High Energy Astrophysics Division (HEAD) for his pioneering work on the development of microcalorimeters that has led to breakthroughs in X-ray astronomy and on soft diffuse X-ray background.
The HEAD Distinguished Career Prize is awarded at the time of the Division Meeting to recognize an individual high-energy astrophysicist who has made outstanding contributions to the field of high energy astrophysics throughout their career. Outstanding contributions include a body of important research results (observational, theoretical or experimental) which have led to ground-breaking results in high-energy astrophysics, and/or a career of mentorship to a new generation of high-energy astrophysicists, especially if this mentorship helped to support under-represented or under-resourced scientists and increased the diversity of the HEA community. The winner gives an invited talk at the Divisional Meeting in the award year. The prize carries a cash award of $1500.
AAS announced many 2025 prizes today; the full list can be found at their website.
This post is adapted from the AAS news release and website linked within the text.
Congrats to physics, astronomy-physics, mathematics and french major Caleb Youngwerth on winning the Meeting Award for Undergraduate Student Poster at the Fall 2024 meeting of the Eastern Great Lakes Section of APS!
Youngwerth’s poster, entitled, “Harnessing Molecular Simulation of the DLVO Potential to Engineer New Battery Technologies,” was presented at the meeting held October 18-19 at Marietta College in Ohio. The work was conducted in the chemical and biological engineering group of Prof. Rose Cersonsky.
The award was announced at the meeting and comes with a cash prize.
For more info, read Chemical and Biological Engineering’s story about Caleb: https://engineering.wisc.edu/blog/student-wins-award-for-research-on-colloidal-gels/