Research, teaching and outreach in Physics at UW–Madison
neutrinos
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.
Bringing the Quantum to the Classical: A Hybrid Simulation of Supernova Neutrinos
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.
Lu Lu receives 2023 IUPAP Early Career Scientist Prize
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.”
IceCube shows Milky Way galaxy is a neutrino desert
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The Milky Way galaxy is an awe-inspiring feature of the night sky, dominating all wavelengths of light and viewable with the naked eye as a hazy band of stars stretching from horizon to horizon. Now,
In a June 30 article in the journal Science, the IceCube Collaboration — an international group of more than 350 scientists — presents this new evidence of high-energy neutrino emission from the Milky Way. The findings indicate that the Milky Way produces far fewer neutrinos than the average distant galaxies.
“What’s intriguing is that, unlike the case for light of any wavelength, in neutrinos, the universe outshines the nearby sources in our own galaxy,” says Francis Halzen, a professor of physics at the University of Wisconsin–Madison and principal investigator at IceCube.
The IceCube search focused on the southern sky, where the bulk of neutrino emission from the galactic plane is expected near the center of the galaxy. However, until now, a background of neutrinos and other particles produced by cosmic-ray interactions with the Earth’s atmosphere made it difficult to parse out neutrinos originating from galactic sources — a significant challenge compounded by relatively sparse neutrino production in general.
Congrats to Ke Fang, assistant professor of physics, WIPAC faculty member, and HAWC spokesperson, on earning an NSF CAREER award! CAREER awards are NSF’s most prestigious awards in support of early-career faculty who have the potential to serve as academic role models in research and education and to lead advances in the mission of their department or organization.
Fang’s award is sponsored by the NSF Windows on the Universe: Multimessenger Astrophysics program. In multimessenger astrophysics, scientists search for multiple high energy signals to identify their sources and learn more about the makeup of our universe. WIPAC hosts both the IceCube neutrino telescope and the HAWC gamma ray telescope, and Fang says she is excited to have access to high-quality data from both. In her NSF proposal, she plans to use that data in two ways.
“One is evolving novel data analysis techniques to study the problems that remain outstanding, such as the source of high-energy neutrinos,” Fang says. “The second part is once we have these data analysis results, then we’ll use numerical simulations to understand our observations.”
In addition to an innovative research component, NSF proposals require that the research has broader societal impacts, such as working toward greater inclusion in STEM or increasing public understanding of science. Once again, Fang finds herself well-positioned at WIPAC, where the outreach team has developed Master Classes, a one-day event where high school students come to WIPAC, spend time with scientists, and learn about topics not typically covered in high school physics class. Currently, the students learn about IceCube’s instrumentation and how to analyze the complex detector data.
“The course is already well designed, but from my perspective, I use a lot of numerical simulation in my research, so one thing I proposed to do is that I would design a module that would incorporate some of these modern numerical study techniques into the master class,” Fang says. “The students will now learn how to study physics using supercomputers, using numerical simulations.”
Help IceCube decode signals from outer space in new Citizen Science project
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Every second, about 100 trillion neutrinos pass through your body unnoticed. At the South Pole, the IceCube Neutrino Observatory detects these elusive particles and works to identify their astronomical origins to help unlock mysteries of the universe. Such an undertaking requires a massive amount of data, with one terabyte of data recorded daily by IceCube. But organizing the data can be labor intensive. This is where the public can help.
Starting today, volunteers from anywhere can participate in the Name that Neutrino project led by IceCube researchers at Drexel University, which asks users to categorize IceCube data. Through the Zooniverse platform, volunteers can join in from the convenience of their own computer or phone. Name that Neutrino is open to everyone and will run for about 10 weeks.
Click “Tutorial” to learn about how to classify signals.
Watch the brief video and pick one of the five categories for signals.
Check out the “Field Guide” for more examples and information.
UW–Madison physicists key in revealing neutrinos emanating from galactic neighbor with a gigantic black hole
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On Earth, billions of subatomic particles called neutrinos pass through us every second, but we never notice because they rarely interact with matter. Because of this, neutrinos can travel straight paths over vast distances unimpeded, carrying information about their cosmic origins.
Although most of these aptly named “ghost” particles detected on Earth originate from the Sun or our own atmosphere, some neutrinos come from the cosmos, far beyond our galaxy. These neutrinos, called astrophysical neutrinos, can provide valuable insight into some of the most powerful objects in the universe.
For the first time, an international team of scientists has found evidence of high-energy astrophysical neutrinos emanating from the galaxy NGC 1068 in the constellation Cetus.
The detection was made by the National Science Foundation-supported IceCube Neutrino Observatory, a 1-billion-ton neutrino telescope made of scientific instruments and ice situated 1.5-2.5 kilometers below the surface at the South Pole.
These new results, to be published tomorrow (Nov. 4, 2022) in Science, were shared in a presentation given today at the Wisconsin Institute for Discovery.
“One neutrino can single out a source. But only an observation with multiple neutrinos will reveal the obscured core of the most energetic cosmic objects,” says Francis Halzen, a University of Wisconsin–Madison professor of physics and principal investigator of the IceCube project. “IceCube has accumulated some 80 neutrinos of teraelectronvolt energy from NGC 1068, which are not yet enough to answer all our questions, but they definitely are the next big step toward the realization of neutrino astronomy.”
Back in 2013, the IceCube Neutrino Observatory—a cubic-kilometer neutrino detector embedded in Antarctic ice—announced the first observation of high-energy (above 100 TeV) neutrinos originating from outside our solar system, spawning a new age in astronomy. Four years later, on September 22, 2017, a high-energy neutrino event was detected coincident with a gamma-ray flare from a cosmic particle accelerator, a blazar known as TXS 0506+056. The coincident observation provided the first evidence for an extragalactic source of high-energy neutrinos.
The identification of this source was possible thanks to IceCube’s real-time high-energy neutrino alert program, which notifies the community of directions and energies of individual neutrinos that are most likely to have come from astrophysical sources. These alerts trigger follow-up observations of electromagnetic waves from radio up to gamma-ray, aimed at pinpointing a possible astrophysical source of high-energy neutrinos. However, the sources of the vast majority of the measured diffuse flux of astrophysical neutrinos still remain a mystery, as do how many of those sources exist. Another mystery is whether the neutrino sources are steady or variable over time and, if variable, whether they vary over long or short time scales.
In a paper recently submitted to The Astrophysical Journal, the IceCube Collaboration presents a follow-up search that looked for additional, lower-energy events in the direction of the high-energy alert events. The analysis looked at low- and high-energy events from 2011-2020 and was conducted to search for the coincidence in different time scales from 1,000 seconds up to one decade. Although the researchers did not find an excess of low-energy events across the searched time scales, they were able to constrain the abundance of astrophysical neutrino sources in the universe.
Map of high-energy neutrino candidates (“alert events”) detected by IceCube. The map is in celestial coordinates, with the Galactic plane indicated by a line and the Galactic center by a dot. Two contours are shown for each event, for 50% and 90% confidence in the localization on the sky. The color scale shows the “signalness” of each event, which quantifies the likelihood that each event is an astrophysical neutrino rather than a background event from Earth’s atmosphere. Credit: IceCube Collaboration
This research also delves into the question of whether the astrophysical neutrino flux measured by IceCube is produced by a large number of weak sources or a small number of strong sources. To distinguish between the two possibilities, the researchers developed a statistical method that used two different sets of neutrinos: 1) alert events that have a high probability of being from an astrophysical source and 2) the gamma-ray follow-up (GFU) sample, where only about one to five out of 1,000 events per day are astrophysical.
“If there are a lot of GFU events in the direction of the alerts, that’s a sign that neutrino sources are producing a lot of detectable neutrinos, which would mean there are only a few, bright sources,” explained recent UW–Madison PhD student Alex Pizzuto, a lead on the analysis who is now a software engineer at Google. “If you don’t see a lot of GFU events in the direction of alerts, this is an indication of the opposite, that there are many, dim sources that are responsible for the flux of neutrinos that IceCube detects.”
Constraints on the luminosity (power) of each individual source as a function of the number density of astrophysical neutrino sources (horizontal axis). Previous IceCube measurements of the total astrophysical neutrino flux indicate that the true combination of the two quantities must lie within the diagonal band marked “diffuse.” The results of the new analysis are shown as an upper limit, compared to the sensitivity, which shows the range of results expected from background alone (no additional signal neutrinos associated with the directions of alert events). The upper limit is above the sensitivity because there is a statistical excess in the result (p = 0.018). Credit: IceCube Collaboration
They interpreted the results using a simulation tool called FIRESONG, which looks at populations of neutrino sources and calculates the flux from each of these sources. The simulation was then used to determine if the simulated sources might be responsible for producing a neutrino event.
“We did not find a clear excess of low-energy events associated with the high-energy alert events on any of the three time scales we analyzed,” said Justin Vandenbroucke, a physics professor at UW–Madison and colead of the analysis. “This implies that there are many astrophysical neutrino sources because, if there were few, we would detect additional events accompanying the high-energy alerts.”
Future analyses will take advantage of larger IceCube data sets and higher quality data from improved calibration methods. With the completion of the larger next-generation telescope, IceCube-Gen2, researchers will be able to detect even more dim neutrino sources. Even knowing the abundance of sources could provide important constraints on the identity of the sources.
“The future is very exciting as this analysis shows that planned improvements might reveal more astrophysical sources and populations,” said Abhishek Desai, postdoctoral fellow at UW–Madison and co-lead of the analysis. “This will be due to better event localization, which is already being studied and should be optimized in the near future.”
+ info “Constraints on populations of neutrino sources from searches in the directions of IceCube neutrino alerts,” The IceCube Collaboration: R. Abbasi et al. Submitted to TheAstrophysical Journal. arxiv.org/abs/2210.04930.
Search for neutrino emission associated with LIGO/Virgo gravitational waves
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Gravitational waves (GWs) are a signature for some of the most energetic phenomena in the universe, which cause ripples in space-time that travel at the speed of light. These events, spurred by massive accelerating objects, act as cosmic messengers that carry with them clues to their origins. They are also probable sources for highly energetic neutrinos, nearly massless cosmic messengers hurtling through space unimpeded. Because neutrinos rarely interact with surrounding matter, they can reveal phenomena that are otherwise unobserved with electromagnetic waves. These high-energy neutrinos are detected by the IceCube Neutrino Observatory, a cubic-kilometer detector enveloped in Antarctic ice at the South Pole.
Both GWs and neutrinos are recently introduced messengers in astronomy and have yet to be detected by the same source. Such a major discovery would not only shed light on the sources of cosmic rays but would also help in understanding the most energetic processes in the universe. By coordinating traditional observations (from radio to gamma rays) with these new messengers, researchers can gain deeper insights into astrophysical sources that were unobtainable before.
Previously, the IceCube Collaboration looked for joint emission of GWs and high-energy neutrinos with data collected by IceCube, the Laser Interferometer Gravitational-Wave Observatory (LIGO), and the Virgo gravitational wave detector. These results were from GWs observed during the first two observing runs (O1 and O2) of LIGO and Virgo. IceCube researchers from the University of Wisconsin–Madison and Columbia University conducted an updated analysis of GWs from the third observing run (O3) of the LIGO/Virgo detectors. The increased number of GWs improved the researchers’ overall analysis. Their findings were recently submitted to TheAstrophysical Journal.
