Research, teaching and outreach in Physics at UW–Madison
Year: 2025
Nuclear physicist Paul Quin has passed away
Posted on
Paul Quin
Emerit professor of physics Paul Quin passed away on October 9, 2025. He was 84.
Born in Brooklyn, NY in 1941, Quin received his doctorate in physics from the University of Notre Dame, where his thesis work centered on the spectroscopy of the SD-shell nuclei. He joined the nuclear physics group at UW–Madison as a postdoc in 1969, playing a central role in the construction and installation of the new Lamb-Shift polarized ion source. He was also one of three survivors of the 1970 Sterling Hall bombing.
Quin joined the faculty in 1971. His research focused on the use of polarized beams as a tool for nuclear spectroscopy and his group made numerous important contributions in this field. In addition, Quin was an important player in the many instrumentation development projects that took place in the nuclear physics lab during the 1970s and the early 80s. In particular, he was the leader of the first experiment to test storage-cell technology for targets of polarized hydrogen atoms, a technology which has gone on to become important for polarization experiments at storage ring machines throughout the world.
Around 1980, Quin began expanding his research focus, moving into the field of weak interactions. In the years that followed, he carried out a variety of interesting and important experiments on β decay of polarized nuclei. These experiments typically involved tests of the conserved-vector-current hypothesis or searches for right-handed currents. In 1986, he and T. Girard published an important paper which described a new and potentially very sensitive technique for detecting right-handed currents in β decay. This new concept, which involves measuring the polarized-nucleus beta asymmetry correlation, became the basis for a number of experiments performed over the subsequent decade in both the U.S. and Europe, with Quin playing a central role in many cases.
Later in his career, Quin continued to work in the area of weak interactions, helping to define the role of various nuclear physics experiments that place constraints on extensions of the standard model. Quin retired in 2001.
Quin also made many contributions to the teaching mission of the department. His great enthusiasm for teaching was always evident, and he frequently introduced new and innovative ideas in the classes he taught. In the ‘80s, he took responsibility for developing new experiments for the Physics 321 lab and upgraded a number of the existing experiments. Towards the end of his teaching career, he was a tireless instructor in the large introductory courses, contributing in a number of important ways to the implementation of computer-based laboratories. In addition, Quin was a staunch supporter of the department’s then-new Peer Mentor Tutor Program. He also supervised nine students who received doctorates under his guidance.
In retirement Paul was an active participant in many feeding projects for those in need. He was instrumental in securing volunteers and food for the Men’s Shelter, The Adopt-a-School program and the Allied Community. In addition he was a long time volunteer for Meals on Wheels. After moving to Maryland to be nearer to his daughter and her family, he was a proud grandparent who enjoyed being with his two grandchildren.
This post was mostly derived from department archives
“Rival” neutrino experiments NOvA and T2K publish first joint analysis
Posted on
The combined results add to physicists’ understanding and validate the impressive collaborative effort between two competing — yet complementary — experiments.
This story was published by Fermilab
When the universe began, physicists expect there should have been equal amounts of matter and antimatter. But if that were so, the matter and antimatter should have perfectly canceled each other out, resulting in total annihilation.
And yet, here we are. Somehow, matter won out over antimatter — but we still don’t know how or why.
Physicists suspect the answer may lie in the mysterious behavior of abundant yet elusive particles called neutrinos. Specifically, learning more about a phenomenon called neutrino oscillation — in which neutrinos change types, or flavors, as they travel — could bring us closer to an answer.
The international collaborations representing two neutrino experiments, NOvA in the United States and T2K in Japan, recently combined forces to produce their first joint results, published October 22 in the journal Nature. This initial joint analysis provides some of the most precise neutrino-oscillation measurements in the field. The NOvA collaboration, centered at Fermilab, includes University of Wisconsin–Madison physicists in Brian Rebel’s group.
“These results are an outcome of a cooperation and mutual understanding of two unique collaborations, both involving many experts in neutrino physics, detection technologies and analysis techniques, working in very different environments, using different methods and tools,” says T2K collaborator Tomáš Nosek.
Caption: T2K in Japan and NOvA in the United States are both long-baseline experiments: they each shoot an intense beam of neutrinos that passes through both a near detector close to the neutrino source and a far detector hundreds of kilometers away. Both experiments compare data recorded in each detector to learn about neutrinos’ behavior and properties. | Credit: Fermilab
Different experiments, common goals
Despite their ubiquity, neutrinos are very difficult to detect and study. Even though they were first seen in the 1950s, the ghostly particles remain deeply enigmatic. Filling in gaps in our knowledge about neutrinos and their properties may reveal fundamental truths about the universe.
T2K and NOvA are both long-baseline experiments: they each shoot an intense beam of neutrinos that passes through both a near detector close to the neutrino source and a far detector hundreds of miles away. Both experiments compare data recorded in each detector to learn about neutrinos’ behavior and properties.
NOvA, the NuMI Off-axis νe Appearance experiment, sends a beam of neutrinos 810 kilometers from its source at the U.S. Department of Energy’s Fermi National Accelerator Laboratory near Chicago, Illinois, to a 14,000-ton liquid-scintillator detector in Ash River, Minnesota.
The T2K experiment’s neutrino beam travels 295 kilometers from Tokai to Kamioka — hence the name T2K. Tokai is home to the Japan Proton Accelerator Research Complex (J-PARC) and Kamioka hosts the Super-Kamiokande neutrino detector, an enormous tank of ultrapure water located a kilometer underground.
Since the experiments have similar science goals but different baselines and different neutrino energies, physicists can learn more by combining their data.
“By making a joint analysis, you can get a more precise measurement than each experiment can produce alone,” says NOvA collaborator Liudmila Kolupaeva. “As a rule, experiments in high-energy physics have different designs even if they have the same science goal. Joint analyses allow us to use complementary features of these designs.”
As long-baseline experiments, NOvA and T2K are ideal for studying neutrino oscillations, a phenomenon that can provide insight into open questions like charge-parity violation and the neutrino mass ordering. Two experiments with different baselines and energies have a better chance of disentangling the two effects than one experiment alone.
Interrogating neutrino oscillations
The mystery of neutrino mass ordering is the question of which neutrino is the lightest. But it isn’t as simple as placing particles on a scale. Neutrinos have miniscule masses that are made up of combinations of mass states. There are three neutrino mass states, but, confusingly, they don’t map to the three neutrino flavors. In fact, each flavor is made of a mix of the three mass states, and each mass state has a different probability of acting like each flavor of neutrino.
There are two possible mass orderings, called normal or inverted. Under the normal ordering, two of the mass states are relatively light and one is heavy, while the inverted ordering has two heavier mass states and one light.
In the normal ordering, there is an enhanced probability that muon neutrinos will oscillate to electron neutrinos but a lower probability that muon antineutrinos will oscillate to electron antineutrinos. In the inverted ordering, the opposite happens. However, an asymmetry in the neutrinos’ and antineutrinos’ oscillations could also be explained if neutrinos violate CP symmetry — in other words, if neutrinos don’t behave the same as their antimatter counterparts.
The combined results of NOvA and T2K do not favor either mass ordering. If future results show the neutrino mass ordering mass ordering is normal, NOvA’s and T2K’s results are less clear on CP symmetry, requiring additional data to clarify. However, if the neutrino mass ordering is found to be inverted, the results published today provide evidence that neutrinos violate CP symmetry, potentially explaining why the universe is dominated by matter instead of antimatter.
“Neutrino physics is a strange field. It is very challenging to isolate effects,” says Kendall Mahn, co-spokesperson for T2K. “Combining analyses allows us to isolate one of these effects, and that’s progress.”
The combined analysis does provide one of the most precise values of the difference in mass between neutrino mass states, a quantity called Δ . With an uncertainty below 2%, the new value will enable physicists to make precision comparisons with other neutrino experiments to test whether the neutrino oscillation theory is complete.
What’s next
These first joint results do not definitively solve any mysteries of neutrinos, but they do add to physicists’ knowledge about the particles. Plus, they validate the impressive collaborative effort between two competing — yet complementary — experiments.
The NOvA collaboration consists of more than 250 scientists and engineers from 49 institutions in eight countries. The T2K collaboration has more than 560 members from 75 institutions in 15 countries. The two collaborations began active work on this joint analysis in 2019; it combines six years of data from NOvA, which began collecting data in 2014, and a decade of data from T2K, which started up in 2010. Both experiments continue to take data, and efforts are already underway to update the joint analysis with the new data.
“The joint analysis work has benefited both collaborations,” says Patricia Vahle, co-spokesperson for NOvA. “We have a much better mutual understanding of the strengths and challenges of the different experimental setups and analysis techniques.”
NOvA and T2K are the only currently operating long-baseline neutrino experiments. Their initial combined results lay a foundation for forthcoming neutrino experiments that will answer the questions around neutrinos unambiguously.
The Fermilab-led Deep Underground Neutrino Experiment is under construction in Illinois and South Dakota in the U.S. With its longer baseline of 1,800 kilometers, DUNE will be more sensitive to neutrino mass ordering and could give physicists a conclusive answer shortly after it turns on in the early years of the next decade.
In Japan, Hyper-Kamiokande, a sequel to Super-Kamiokande located beneath a mountain in Hida City, will be more sensitive to CP violation. And a medium-baseline reactor neutrino experiment in China called JUNO recently began additional studies of antineutrinos and their behavior. Two experiments that use neutrinos generated in the atmosphere to study oscillations, KM3Net-Orca and IceCube, also continue to take data.
Many physicists hope these next-generation neutrino experiments can come together — as NOvA and T2K have already done — to make progress on their shared scientific goals to learn more about neutrinos and their unusual properties.
“As shown in this very analysis, there are no truly ‘rivaling’ experiments because they all share a common goal of scientific study of a phenomenon,” says Nosek. “Collaborating is naturally important for the transfer of knowledge, know-how and experience, and for sharing resources, ideas and tools. The T2K-NOvA collaboration is not merely a sum of T2K and NOvA collaborations. It is much, much more.”
UW fostering closer research ties with federal defense, cybersecurity agencies
UW–Madison leaders seek to expand partnership with federal agencies to boost dual-use research funding.
Exploring Decades of Semiconductor Collaboration between Argonne National Lab & UW–Madison
UW–Madison and Argonne National Laboratory have built a portfolio of shared research for decades. Read how semiconductor researchers from all interest areas have benefited from this affiliation.
Deniz Yavuz elected Fellow of the American Physical Society
Posted on
Deniz Yavuz
Congratulations to Prof. Deniz Yavuz, who was elected a 2025 Fellow of the American Physical Society!
He was elected “for outstanding experimental and theoretical contributions to nanoscale localization of atoms with electromagnetically induced transparency and collective radiation effects in atomic ensembles,” and nominated by the Division of Atomic, Molecular & Optical Physics (DAMOP).
APS Fellowship is a distinct honor signifying recognition by one’s professional peers for outstanding contributions to physics. Each year, no more than one half of one percent of the Society’s membership is recognized by this honor.
Congrats to Vladimir Zhdankin, assistant professor of physics, on earning a Department of Energy Early Career award! The five-year award will fund his research on energy and entropy in collisionless, turbulent plasmas.
Systems in equilibrium are easy to describe, but often the most interesting questions in nature are complex and dynamic. Most plasmas, including astrophysical ones and manmade ones on earth, are not in equilibrium, so they are more difficult to characterize. Zhdankin’s research is working toward a more universal understanding of non-equilibrium plasmas, in the form of mathematical equations that can then be broadly applied.
“We think that our understanding of plasmas isn’t finished yet, and there are still some basic ingredients in the statistical mechanics which, once we understand better, we’ll have a more predictive framework for how plasmas should behave,” Zhdankin says.
Collisionless plasmas have a low enough particle density where the particles largely flow without bumping into each other. Instead, their trajectories are controlled by the electric and magnetic field, which leads to a generally chaotic flow, like the rapids of a river. It is that dynamic turbulence that causes these plasmas to be non-equilibrium, leading to interesting, if not straightforward, properties.
“In these systems, energy is conserved — it has to be,” Zhdankin says. “But we don’t quite have a handle on what’s happening with the entropy. We have reason to believe it’s increasing, consistent with the second law of thermodynamics, but it doesn’t seem to reach a maximum.”
Zhdankin’s goal is to better understand the energy and entropy in these complex plasmas through “particle-in-cell” simulations, where tens of billions of plasma particles — electrons and protons — are simulated in a small box, then manipulated in various ways.
“We imagine stirring the plasma to make it more turbulent and putting some energy into it, and then we want to see how it heats up and how the particles achieve higher energies,” Zhdankin says. “What if we increase or decrease the size of the box? Make the magnetic field stronger? Make the particles collide a little bit?”
The simulations can then be compared to real-world data, including measurements of the solar wind or laboratory plasmas. An ideal outcome would be obtaining formulae that better describe these complex, turbulent plasmas and can be applied across a broad range of systems, from laboratory experiments to the accretion flows of black holes.
“And there’s a chance we’re just not going to be able to get something predictive out of this work, if there’s just too big of a landscape of possibilities,” Zhdankin says. “But this topic, I consider it one of the most fundamental ones that could be studied in plasma physics.”
With major U.S. investment, UW-Madison leads effort to advance abundant fusion energy for all
Double the Higgs, Double the Mystery! The hunt for a new, heavy particle decaying to a pair of Higgs Bosons
Posted on
This story, written by physics grad student Ganesh Parida, was originally published by the CMS collaboration
CMS scientists are on the hunt for a new, heavy particle that decays into a pair of Higgs bosons. Using the final state with two bottom quarks and two tau leptons, the search sets the most stringent limits to date in the mass range 1.4–4.5 TeV.
Ganesh Parida
The CMS experiment is searching for signs of new, heavy particles that could decay into pairs of Higgs bosons – we call this an HH signature. These signatures are particularly exciting because they can give us clues about the stability of our universe and open a window to physics beyond our current understanding of fundamental particles and their interactions, the standard model.
In this search, we focus on a final state where one Higgs boson decays to two bottom quarks (H→bb) and the other decays to two tau leptons (H→ττ). This final state offers a promising balance: it has a relatively large probability of occurring, while also allowing us to separate signal events from background processes. Performing such a search is far from straightforward. If a new heavy particle were produced at the LHC, it would impart a large momentum, a “boost”, to its daughter Higgs bosons. The boost causes the decay products of each Higgs boson to be collimated and overlap in the detector, making their reconstruction quite challenging.
Diagram showing a new physics process explored in this search. Two protons collide and produce a new heavy particle X, which then decays into two standard model Higgs bosons , which in turn give two bottom quarks and two tau leptons in the final state. | Credit: CMS Collaboration
To meet this challenge, CMS uses advanced reconstruction and machine-learning techniques. For the H→bb decay, the bottom quarks form collimated sprays of particles, called jets, which overlap to a large extent. To identify them, a graph neural network, called ParticleNet, is trained to recognize the pattern of the two bottom quark jets inside a single, large jet.
Reconstructing the H→ττ is a two-step process: first, we untangle and reconstruct the two really close taus, and then we use a convolutional neural network, called Boosted DeepTau to figure out the characteristics of these reconstructed taus and tell them apart from background jets. Because tau leptons also produce invisible neutrinos, we apply a likelihood-based method to obtain the four-momentum of the parent Higgs boson.
Once both Higgs bosons are reconstructed, we can combine them to measure the mass of the system. If a new heavy particle exists, it would appear as a peak, or “bump,” on top of the smoothly falling background distribution. This strategy is often referred to as a “bump hunt” – a classic tool in the search for new particles at colliders.
Left: The sketch illustrates how we perform a “bump hunt.” Background processes fall smoothly with increasing mass, while a new particle would create a visible peak on top of this distribution. Right: We reconstruct the mass of Higgs boson pairs from collision data and compare it to standard model background predictions (shown in color). The black points show the recorded data, while the dashed lines illustrate how new heavy particles could appear. The data follow the standard model expectation, and CMS does not observe a significant excess. | Source: CMS Collaboration
After analyzing data from the full LHC Run 2 (2016–2018), CMS did not observe any significant deviation from the standard model prediction. While this means that no new particle was discovered in this final state yet, the analysis sets the most stringent upper limits to date on the possible production of heavy particles decaying into Higgs boson pairs in the bbττ final state in the mass range of 1.4 TeV to 4.5 TeV.
“The results may not yet show evidence of new physics, but they are paving the way,” says Ganesh Parida, a PhD student at the University of Wisconsin–Madison, who carried out this analysis together with Camilla Galloni and Deborah Pinna, both scientists at the University of Wisconsin–Madison and members of CMS. “It has been both exciting and rewarding to learn, develop, and apply sophisticated techniques to probe these challenging boosted regimes.”
The biggest challenge here is the sheer number of events we can collect for these difficult “boosted” scenarios. That is why the ongoing Run 3 and the upcoming High-Luminosity runs of the LHC are so important – they will give us the biggest datasets ever for a potential discovery!
Read the full article at:
