High Energy

NOvA study sets tighter limit on sterile neutrinos

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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?’”

two 3-d plots depicting a neutrino detection at the far detector. On the left, the detections show a scatter of light, on the right, the detections show a long, single trace (wtih a small amount of scatter near the entry point)
Depictions of particle traces at the far detector. In both images, the beam enters from the left and moves through the detector. When a neutrino interacts in the detector, it can produce a number of charged particles, which produce light as they travel through the detector. The light is recorded, and shown here as points. Based on the light pattern, the researchers use a neural network to classify an event as a neutral current interaction (left panel) with a characteristic shower of particles or as a charged current interaction (right), with a characteristic long track. | Provided by Adam LIster

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.”

Profile picture of Adam Lister
Adam Lister
profile photo of Harry Hausner
Harry Hausner
profile photo of Brian Rebel
Brian Rebel

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.

a graph of reconstructed neutrino energy on the x-axis and number of events in a given range of energy on the y-axis.
The number of times a neutrino of a given energy range (x axis) was counted (y axis) at the far detector, shown as data points. The shaded grey area shows where the data should fit if the three-flavor (i.e. no sterile neutrinos) model is true, and overall there are no deviations from this model. The dark shaded area at the bottom shows counts due to cosmic rays. | From this study

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. 

 

Welcome, Prof. Jakob Moritz!

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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!

New NOvA results add to mystery of neutrinos

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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 precision. The data favor the “normal” ordering of neutrino masses more strongly than before, but ambiguity remains around the neutrino’s oscillation properties.

At UW–Madison, the NOvA collaboration includes physics professor Brian Rebel, postdoc Adam Lister, former postdoc Tom Carroll, and grad student Anna Cooleybeck.

The latest NOvA data provide a very precise measurement of the bigger 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 at almost 7:1 odds. This suggests that neutrinos adhere to the normal ordering, but physicists have not met the high threshold of certainty required to declare a discovery.

Read the full story, originally published by Fermilab

Two physics students win presentation awards at APS April Meeting

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Elias Mettner and Nadia Talbi, both conducting research in high energy physics at UW–Madison, won undergraduate presenter awards at the American Physical Society’s April Meeting.

The meeting, held in Sacramento April 3-6, included seven undergraduate oral presentation sessions with six to eight students in each session. The top two students from each session earned “Top Presenter” awards. Mettner and Talbi were the only two UW–Madison students who gave oral presentations, and both won awards.

profile photo of Elias Mettner
Elias Mettner

Mettner is a physics major working with scientist Abdollah Mohammadi. His talk was titled “Pair Production and Hadron Photoproduction Backgrounds at the Cool Copper Collider.”

The Cool Copper Collider is a proposed electron-positron collider that will help scientists to explore the Higgs boson even further. The electron-positron beam will have some natural decay that converts into particles and is recorded by the detector. Mettner’s research asks how this beam background will impact the detector.

“The detector will record this background, and it could take the place of the data we want or make it harder to reconstruct data,” Mettner says. “It’s important to make sure that the backgrounds that will come into the detector using this new design will not cause any issues, otherwise the benefits of this collider design cannot be put to their maximum use.”

Mettner had been interested in physics from a young age and comes from a family of teachers who encouraged him to explore his academic interests. Upon entering UW–Madison, he jumped at the chance to conduct research in particle physics. He joined the UW CMS Collaboration in his freshman year through the Undergraduate Research Scholars program and began his project with the Cool Copper Collider soon after. He was also awarded the Sophomore Research Fellowship for his junior year and the Hilldale Research Fellowship for his upcoming senior year.

a woman stands in front of a screen with a powerpoint presentation title slide showing
Nadia Talbi presents at APS April Meeting

Talbi is an astronomy-physics major working in physics professor Tulika Bose’s group and mentored by postdoc Charis Koraka. Her talk, “A Search for Vector-Like Leptons: Compact Analysis,” covered work she has done through a Thaxton Fellowship.

“Bosons are force particles, and basically every boson except for the Higgs — the photon, the gluon — is a vector boson. Leptons are electrons, muons, neutrinos, stuff like that,” Talbi explains. “Vector-like leptons are a hypothetical particle, we don’t know whether or not they exist.”

Talbi was drawn to astronomy because she has long had an interest in the fundamental nature of the universe. As a child, she read an article on Dark Matter and, later, a friend gave her a book on the Standard Model. She was hooked. When she applied for the Thaxton Fellowship, a departmental program that was started to provide more equitable access to undergraduate research in physics, she discussed her interest in particle physics and the research at CERN, which landed her in Bose’s group.

“So before I even had any formal education in physics, where things can be very black and white, I’ve had the opportunity to understand the beautiful things within the field,” Talbi says. “Studying physics, I think, gives you some of the most fundamental understanding of our existence.”

Both Metter and Talbi say that attending conference was overall a very worthwhile experience — even if they both had to take an E+M exam remotely before presenting. (“It was a good bonding experience,” Talbi says.)

“The conference was a lot of fun, and worth it to go and make some connections and experience a bunch of really interesting research from people all in different stages of their careers,” Mettner says.

Adds Talbi: “There were so many undergraduates there, I met so many, I made a lot of friends. It felt like there was a community.”

Both students were also invited to present their award-winning talks to the Physics Board of Visitors spring meeting.

Tulika Bose honored with Vilas Distinguished Achievement Professorship

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Vilas Distinguished Achievement Professor Tulika Bose

Sixteen professors, including physics professor Tulika Bose, were named to Vilas Distinguished Achievement Professorships, an award recognizing distinguished scholarship as well as standout efforts in teaching and service. The professorship provides five years of flexible funding — two-thirds of which is provided by the Office of the Provost through the generosity of the Vilas trustees and one-third provided by the school or college whose dean nominated the winner. The awards are supported by the estate of professor, U.S. Senator and UW Regent William F. Vilas (1840-1908).

Federal physics advisory panel — including Profs. Bose and Cranmer — announces particle physics recommendations

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Earlier this year, physics professors Tulika Bose and Kyle Cranmer were selected to serve on the Particle Physics Project Prioritization Panel, or P5, a group of High Energy Physics experts that advises the Department of Energy Office of Science and the National Science Foundation’s Division of Physics on high energy and particle physics matters.

P5 announced their recommendations in a draft report published Dec. 7 — and UW–Madison physicists are featured in many of the projects.

One recommendation is to move forward with a planned expansion of the IceCube Neutrino Observatory, an international scientific collaboration operated by the UW–Madison at the South Pole. Other recommendations include support for a separate neutrino experiment based in Illinois (the Deep Underground Neutrino Experiment, or DUNE); continuing investment in the Large Hadron Collider in Switzerland and the Rubin Observatory in Chile; and expanding involvement in the Cherenkov Telescope Array (CTA), a ground-based very-high-energy gamma ray observatory. UW–Madison physicists have leading roles in all of these research efforts.

Additional recommendations include the development of a next generation of ground-based telescopes to observe the cosmic microwave background and a direct dark matter detector experiment, among others.

Read the full story

Wasikul Islam honored with UW Postdoc Association Excellence in Service Award

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Wasikul Islam, a postdoc in Sau Lan Wu’s group, was recognized by the UW–Madison Postdoc Association with an Excellence in Service Award. He was nominated for his science outreach activities, promotion of basic sciences, volunteering and mentorship to undergrad Physics students through various non-profit organizations including the American Physical Society. 

The Postdoc Excellence Awards recognize current postdocs on the UW-Madison campus that contribute their time, knowledge, energy, and enthusiasm to mentoring, teaching, and service. They were established to encourage and reward excellence, innovation, and effectiveness in the mentoring, teaching, and service of UW-Madison postdocs. The 2023 winners were honored at the Celebration of Postdoc Excellence on May 19.

a zoomed out photo shows a man receiving an award from a woman on an elevate stage, with a screen behind them showing his photo, name, and award won
Wasikul Islam receives his award at the 2023 Celebration of Postdoc Excellence, held May 19.

Department of Energy grant to train students at the interface of high energy physics and computer science

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a long row of stacked computer servers

To truly understand our physical world, scientists look to the very small, subatomic particles that make up everything. Particle physics generally falls under the discipline of high energy physics (HEP), where higher and higher energy collisions — tens of teraelectronvolts, or about ten trillion times the energy of visible light — lead to the detection and characterization of particles and how they interact.

These collisions also lead to the accumulation of inordinate amounts of data, and HEP is increasingly becoming a field where researchers must be experts in both particle physics and advanced computing technologies. HEP graduate students, however, rarely enter graduate school with backgrounds in both fields.

Physicists from UW–Madison, Princeton University, and the University of Massachusetts-Amherst are looking to address the science goals of the HEP experiments by training the next generation of software and computing experts with a 5-year, ~$4 million grant from the U.S. Department of Energy (DOE) Office of Science, known as Training to Advance Computational High Energy Physics in the Exascale Era, or TAC-HEP.

“The exascale era is upon us in HEP and the complexity, computational needs and data volumes of current and future HEP experiments will increase dramatically over the next few years. A paradigm shift in software and computing is needed to tackle the data onslaught,” says Tulika Bose, a physics professor at UW–Madison and TAC-HEP principal investigator. “TAC-HEP will help train a new generation of software and computing experts who can take on this challenge head-on and help maximize the physics reach of the experiments.”

Tulika Bose

In total, DOE announced $10 million in funding today for three projects providing classroom training and research opportunities in computational high energy physics to train the next generation of computational scientists and engineers needed to deliver scientific discoveries.

At UW–Madison, TAC-HEP will annually fund four-to-six two-year training positions for graduate students working on a computational HEP research project with Bose or physics professors Keith Bechtol, Kevin Black, Kyle Cranmer, Sridhara Dasu, or Brian Rebel. Their research must broadly fit into the categories of high-performance software and algorithms, collaborative software infrastructure, or hardware-software co-design.

Bose’s research group, for example, focuses on proton-proton collisions in the Compact Muon Solenoid (CMS) at the CERN Large Hadron Collider (LHC). The high luminosity run of the LHC, starting in 2029, will bring unprecedented physics opportunities — and computing challenges, challenges that TAC-HEP graduate students will tackle firsthand.

“The annual data volume will increase by 30 times while the event reconstruction time will increase by nearly 25 times, requiring modernization of the software and computing infrastructure to handle the demands of the experiments,” Bose says. “Novel algorithms using modern hardware and accelerators, such as Graphics Processing Units, or GPUs, will need to be exploited together with a transformation of the data analysis process.”

TAC-HEP will incorporate targeted coursework and specialized training modules that will enable the design and development of coherent hardware and software systems, collaborative software infrastructure, and high-performance software and algorithms. Structured R&D projects, undertaken in collaboration with DOE laboratories (Fermilab and Brookhaven National Lab) and integrated within the program, will provide students from all three participating universities with hands-on experience with cutting-edge computational tools, software and technology.

The training program will also include student professional development including oral and written science communication and cohort-building activities. These components are expected to help build a cohort of students with the goal of increasing recruitment and retention of a diverse group of graduate students.

“Future high energy physics discoveries will require large accurate simulations and efficient collaborative software,” said Regina Rameika, DOE Associate Director of Science for High Energy Physics. “These traineeships will educate the scientists and engineers necessary to design, develop, deploy, and maintain the software and computing infrastructure essential for the future of high energy physics.