Research, teaching and outreach in Physics at UW–Madison
particle physics
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.
Entangled neutrinos may lead to heavier element formation
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Elements are the building blocks of every chemical in the universe, but how and where the different elements formed is not entirely understood. A new paper in The Astrophysical Journal by University of Wisconsin–Madison physics professor Baha Balantekin and colleagues with the Network for Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Physics Frontier Center, shows how entangled neutrinos could be required for the formation of elements above approximately atomic number 140 via neutron capture in an intermediate-rate process, or i-process.
Baha Balantekin
Why it’s important
“Where the chemical elements are made is not clear, and we do not know all the possible ways they can be made,” Balantekin says. “We believe that some are made in supernovae explosions or neutron star mergers, and many of these objects are governed by the laws of quantum mechanics, so then you can use the stars to explore aspects of quantum mechanics.”
What is already known?
Immediately after the Big Bang, lighter elements like hydrogen and helium were abundant. Heavier elements, up to iron (atomic number 26) continued to form through nuclear fusion in the centers of hot stars.
Above iron, fusion is no longer energetically favorable, and nuclear synthesis occurs via neutron capture, where neutrons glom onto atomic nuclei. At high enough concentrations, neutrons can convert into protons, increasing the atomic number of the element by one.
This conversion is dependent on neutrinos and antineutrinos. Neutron capture has been found to occur slowly (s-process, over years) and rapidly (r-process, within minutes); an intermediate timescale, or i-process has been proposed but little evidence exists to support it. Rapid or intermediate neutron capture can only take place in catastrophic events where a huge amount of energy is released, such as supernova collapse.
“When a supernova collapse occurs, you start with a big star, which is gravitationally bound, and that binding has energy,” Balantekin says. “When it collapses, that energy has to be released, and it turns out that energy is released in neutrinos.”
The laws of quantum mechanics state that those neutrinos can become entangled because they interact in the collapsing supernova. Entanglement is when any two or more particles interacted and then “remember” the others, no matter how far apart they might be.
A quick summary of the research
“One question we can ask is if these neutrinos are entangled with each other or not,” Balantekin says. “This paper shows that if the neutrinos are entangled, then there is an enhanced new process of element production, the i-process.”
The abundance pattern based on calculations in this paper (ν i-process pattern; purple line), compared with the solar system s-process (gray line) and r-process (black line) abundance data (Sneden et al. 2008). The ν i abundance for A = 143 is scaled to the solar r-process data for pattern comparison. | Source: The Astrophysical Journal
The experimental and simulated evidence
The researchers used two known facts to set up their calculations: well-established rates of neutron capture, and catalogs of the atomic spectra of stars, which astronomers have collected over decades to identify the abundance of different elements. They also knew that a supernova collapse produces on the order of 10^58 neutrinos, a number that is far too large to use in any standard calculations.
Instead, they made simulations of up to eight neutrinos and calculated the abundance of elements that would be created via neutron capture if the neutrinos were entangled, or were not entangled.
“We have a system of, say, three neutrinos and three antineutrinos together in a region where there are protons and neutrons and see if that changes anything about element formation,” Balantekin says. “We calculate the abundances of elements that are produced in the star, and you see that the entangled or not entangled cases give you different abundances.”
The simulations showed that elements with atomic number greater than 140 are likely to be enhanced by i-process neutron capture — but only if the neutrinos are entangled.
Caveats and future work
Balantekin points out that these simulations are just “hints” based on astronomical observations. Astrophysics research requires using the cosmos as a lab, and it is difficult to conduct true experimental tests on earth.
“There’s something called the standard model of particle physics, which determines the interaction of particles. The neutrino-neutrino interaction is one aspect of the standard model which has not been tested in the lab, it can only be tested in astrophysical extremes,” Balantekin says. “But other aspects of the standard model have been tested in the lab, so one believes that it should all work.”
The researchers are currently using more astrophysical data of element abundance in extreme environments to see if those abundances continue to be explained by entangled neutrinos.
This research is supported in part by the National Science Foundation grants Nos. PHY-1630782 and PHY-2020275 (Network for Neutrinos, Nuclear Astrophysics and Symmetries). Balantekin is supported in part by the U.S. Department of Energy, Office of Science, Office of High Energy Physics, under Award No. DE-SC0019465 and in part by the National Science Foundation Grant PHY-2108339 at the University of Wisconsin-Madison.
The paper’s co-authors include Michael Cervia, Amol Patwardhan, Rebecca Surman, and Xilu Wang, all current or former members of N3AS.
Ke Fang named inaugural recipient of the Bernice Durand Faculty Fellowship
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The Department of Physics is pleased to announce that Ke Fang, assistant professor of physics and WIPAC investigator, has received the inaugural Bernice Durand Faculty Fellowship. This fellowship, given in honor of late Professor Emerit of Physics Bernice Durand, recognizes Fang’s major contributions to the analysis of data from the NASA Fermi satellite, the High Altitude Water Cherenkov (HAWC) telescope and IceCube, and for fundamental theoretical insights in their multimessenger context. Fang is a Sloan Fellow, has been awarded an NSF CAREER award, and is the spokesperson for the HAWC experiment.
Department Chair and professor Mark Eriksson (left) presents assistant professor Ke Fang with the Bernice Durand Faculty Fellowship at the department awards banquet in May 2024.
Durand was one of the first two women professors in the UW–Madison Department of Physics. While at UW–Madison, Durand was a theoretical physicist who specialized in particle theory and mathematical physics. Her research was on symmetry relations in algebra and physics, plus the phenomenology of high-energy interactions at large particle accelerators.
As the first Associate Vice Chancellor for Diversity & Climate, Professor Durand provided leadership to ensure that faculty, staff, and student diversity issues including race, ethnicity, gender, sexual preference, and classroom and general campus workplace climate issues be addressed, and that search committees for non-classified staff be trained in broadening the pool of applicants and eliminating implicit bias. Durand co-directed a grant from the Alfred P. Sloan Foundation to the UW System designed to create more equity, flexibility and career options for faculty and academic staff. She was also a member of the leadership team of the Women in Science and Engineering Leadership Institute sponsored by the National Science Foundation to increase the participation and status of women in science.
A recipient of the Chancellor’s Award for Outstanding Teaching, Professor Durand taught courses at all levels, from modern physics for non-scientists (“Physics for Poets”) to a specialized course she developed for advanced graduate students in the use of topology and algebra in quantum field theory. In the mid 1990s, she used technological and pedagogical techniques in her teaching, such as broadcasting her modern physics for non-scientists course on public television with web-based coursework, and pioneering one of two early versions of MOOCs (massive open online courses) on campus.
The Bernice Durand Faculty Fellowship was conceived by our Board of Visitors, who spearheaded the ultimately-successful fundraising effort, with support from Professor Emerit Randy Durand for this fellowship honoring his wife.
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.
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.
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.
UW–Madison physicist Francis Halzen elected to National Academy of Sciences
Halzen directs the UW–Madison Institute for Elementary Particle Physics Research and is the principal investigator of the IceCube Neutrino Observatory.
Simulating quantum systems on classical computers is currently a near-impossible task, as memory and computation time requirements scale exponentially with the size of the system. Quantum computers promise to solve this scalability issue, but there is just one problem: they can’t reliably do that right now because of exorbitant amounts of noise.
So when UW–Madison physics postdoc Pooja Siwach, former undergrad Katie Harrison BS ‘23, and professor Baha Balantekin wanted to simulate neutrino evolution inside a supernova, they needed to get creative.
Pooja Siwach
Their focus was on a phenomenon called collective neutrino oscillations, which describes a peculiar type of interaction between neutrinos. Neutrinos are unique among elementary particles in that they change type, or flavor, as they propagate through space. These oscillations between flavors are dictated by the density of neutrinos and other matter in the medium, both of which change from the core to the outer layers of a supernova. Physicists are interested in how the flavor composition of neutrinos evolve in time; this is calculated using a time evolution simulation, one of the most popular calculations currently done on quantum computers.
Ideally, researchers could calculate each interaction between every possible pair of neutrinos in the system. However, supernovae produce around 10^58 neutrinos, a literally astronomical number. “It’s really complex, it’s very hard to solve on classical computers,” Siwach says. “That’s why we are interested in quantum computing because quantum computers are a natural way to map such problems.”
Katie Harrison
This naturalness is due to the “two-level” similarities between quantum computers and neutrino flavors. Qubits are composed of two-level states, and neutrino flavor states are approximated as two levels in most physical systems including supernovae.
In a paper published in Physical Review D in October, Siwach, Harrison, and Balantekin studied the collective oscillation problem using a quantum-assisted simulator, or QAS, which combines the benefits of the natural mapping of the system onto qubits and classical computers’ strength in solving matrix equations.
In QAS, the interactions between particles are broken down into a linear combination of products of Pauli matrices, which are the building blocks for quantum computing operations, while the state itself is split into a sum of simpler states. The quantum portion of the problem then boils down to computing products of basis states with each Pauli term in the interaction. These products are then inputted into the oscillation equations.
Flavor composition (y-axis) of four supernova neutrinos over time due to collective oscillations, calculated using the quantum-assisted simulator. The change in flavor for each neutrino over time shows the effect of neutrino-neutrino interactions.
“Then we get the linear-algebraic equations to solve, and solving such equations on a quantum computer requires a lot of resources,” explains Siwach. “That part we do on classical computers.”
This approach allows researchers to use the quantum computers only once before the actual time evolution simulation is done on a classical computer, avoiding common pitfalls in quantum calculations such as error accumulation over the length of the simulation due to noisy gates. The authors showed that the QAS results for a four-neutrino system match with a pure classical calculation, showcasing the power of this approach, especially compared to a purely quantum simulation which quickly deviates from the exact solution due to accumulated errors from gates controlling two qubits at the same time.
Still, as with any current application of quantum computers, there are limitations. “There’s only so much information that we can compute in a reasonable amount of time [on quantum computers],” says Siwach. She also laments the scalability of both the QAS and full quantum simulation. “One more hurdle is scaling to a larger number of neutrinos. If we scale to five or six neutrinos, it will require more qubits and more time, because we have to reduce the time step as well.”
Harrison, who was an undergraduate physics student at UW–Madison during this project, was supported by a fellowship from the Open Quantum Initiative, a new program to expand undergrad research experiences in quantum computing and quantum information science. She enjoyed her time in the program and thinks that it benefits students looking to get involved in research in the field: “I think it’s really good for students to see what it really means to do research and to see if it’s something that you’re capable of doing or something that you’re interested in.”
Flavor composition of a neutrino over time using a full quantum simulation (red points) compared to exact solution (black line). The points start to drift from the exact solution after only a few oscillations, highlighting how noise in the quantum computer negatively affects the calculation.
Vernon Barger elected AAAS Fellow
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This story is modified from one published by University Communications
Barger was elected for “seminal work in studying fundamental particles at colliders and leadership in particle phenomenology, where theory meets experiment.”
This year, 502 scientists, engineers and innovators were chosen from the AAAS membership to be AAAS Fellows. The honor, presented annually since 1874, recognizes efforts to advance science and society, with the fellows chosen to reflect the highest standards of scientific integrity and professional ethics.
“As we celebrate the 150th anniversary of the AAAS Fellows, AAAS is proud to recognize the newly elected individuals,” said Sudip S. Parikh, AAAS chief executive officer and executive publisher of the Science family of journals. “This year’s class embodies scientific excellence, fosters trust in science throughout the communities they serve and leads the next generation of scientists while advancing scientific achievements.”
The new class of fellows will be featured in the April issue of the journal Science, and each new fellow will be celebrated at a September event in Washington, D.C.
Wasikul Islam wins 2024 WISL award for communicating science
Wasikul Islam, a postdoctoral researcher with Sau Lan Wu’s group
For this award, postdocs write a short article that describes their research for the general public. Interested postdocs apply to submit an article, then receive guidelines and editorial assistance to improve accessibility. Approved articles are posted on the WISL website, and the author receives a $250 award.
Islam’s article, “Navigating Anomalies in the Pursuit of New Physics” shares how particle physicists study the fundamental building blocks of the universe, exploring the complex behaviors of subatomic particles that shape our world.
“As a postdoctoral researcher, my journey is not limited to research alone,” Islam says in the article. “It comes with an important responsibility to share the wonders and experiences of scientific research with diverse communities.”
The dual mission of the Wisconsin Initiative for Science Literacy is to promote literacy in science, mathematics and technology among the general public and to attract future generations to careers in research, teaching and public service. WISL is directed by Professor Bassam Z. Shakhashiri of the University of Wisconsin-Madison Chemistry Department. Programs draw on the concepts developed by Dr. Shakhashiri during many years of innovative work in science education.
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.
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.
IceCube collaborator and UW–Madison assistant professor of physics Lu Lu received a 2023 International Union of Pure and Applied Physics (IUPAP) Early Career Scientist Prize “for her contributions to the development of high energy neutrino astronomy in the PeV energy region.” Lu accepted the award on July 27 during the opening ceremony at the 38th International Cosmic Ray Conference (ICRC) held in Nagoya, Japan.
Lu Lu
Early Career Scientist Prizes are given to early career scientists within each IUPAP commission who have up to eight years of postdoctoral research experience and have made significant contributions to the cosmic ray field. Lu is a recipient of the Early Career Scientist Prize in the Commission on Astroparticle Physics (C4).
Her PhD work focused on developing a novel technique to search for ultra-high-energy photons using data from the Pierre Auger Observatory. She also played a leading role in the initial design of the “Dual optical sensors in an Ellipsoid Glass for Gen2” (D-Egg), a two-PMT optical module for the IceCube Upgrade.
Lu is currently an assistant professor of physics at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at the University of Wisconsin–Madison. Her current research focuses on diffuse high-energy astrophysical/cosmogenic neutrinos from TeV to EeV, Galactic PeVatron detection in the context of multimessenger observations, and the exploration of potential transient ultra-high-energy sources.
She is actively involved in IceCube outreach initiatives and has pioneered the development of an app that provides IceCube real-time alerts via augmented reality on mobile devices. Currently, she serves as co-lead of the diffuse science working group in IceCube and is one of three representatives of the physical science group of US-SCAR (Scientific Committee of Antarctic Research).
“I would like to express my deep appreciation for my collaborators and for those who work on foundational tasks such as reconstructions and calibrations, as their efforts behind the scenes make groundbreaking discoveries possible,” said Lu. “As early career scientists, we bear the responsibility of continuing and expanding experiments in the particle astrophysics field. We must collaborate and work together to ensure that the next generation of young scientists will have exciting discoveries to make.”
