Research, teaching and outreach in Physics at UW–Madison
UW–Madison physicists key in revealing neutrinos emanating from galactic neighbor with a gigantic black hole
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.
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.”
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
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.
The Department of Physics is happy to announce that Professor Brian Rebel has been promoted to full professor.
Rebel is a high energy experimentalist whose research focuses on accelerator-based neutrino physics. He joined the department as an associate professor with a joint appointment at Fermilab in 2018, where he is now a senior scientist.
“Professor Rebel is a leader in neutrino science, making major contributions to DUNE experiments and having published recently on four different neutrino collaborations,” says Mark Eriksson, physics department chair. “The department is thrilled about his promotion to full professor.”
Rebel has established himself as a leader in the Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE). DUNE is an international experiment for neutrino science and proton decay studies that consists of two neutrino detectors — one near Fermilab in Illinois, and one in South Dakota. The experiment will be installed in LBNF, which will produce the neutrino beam. Rebel is currently the DUNE Anode Plane Assembly (APA) consortium manager, and has previously led Fermilab’s DUNE Science Group.
Since 2005, Rebel has also been involved in Fermilab’s NOvA experiment, which uses precision measurements to investigate the flavor oscillations of neutrinos that are not predicted by the Standard Model. He is currently serving as the co-convener of the analysis group searching for oscillations of active neutrino flavors into a sterile neutrino.
Rebel is currently training three graduate students and two postdoctoral scholars, and expects to graduate his first UW–Madison doctoral student soon. Additionally, he supervised several trainees at Fermilab before he came to UW–Madison. He has enjoyed teaching both introductory physics as well as physics courses for non-majors, and is an effective and engaging teacher.
Congrats, Prof. Rebel, on this well-deserved recognition!
The IceCube Neutrino Observatory, a massive astroparticle physics experiment located at the South Pole, will be featured in two upcoming documentaries about neutrinos produced for the BBC and PBS NOVA.
Sometimes called the world’s biggest and strangest telescope, IceCube comprises over 5,000 light sensors deployed in a cubic kilometer of ice at the South Pole. Despite its inhospitable environment, the South Pole’s abundance of ice makes it an ideal location for detecting neutrinos: tiny fundamental particles that could reveal unseen parts of the universe.
For these documentaries, IceCube staff from the experiment’s headquarters at the Wisconsin IceCube Particle Astrophysics Center (WIPAC), a research center of the University of Wisconsin–Madison, captured video footage at the South Pole. During the austral summer of 2019, Kael Hanson, John Hardin, Matt Kauer, John Kelley, and Yuya Makino recorded video at the bottom of the world as they conducted annual maintenance and other work on the observatory. The footage was then sent “up north” for use in the two different documentaries.
The BBC documentary, “Neutrino: Hunting the Ghost Particle,” will premiere on BBC Four on Wednesday, September 22 from 9:00 – 10:00 pm BST. It is described as “an astonishing tale of perseverance and ingenuity that reveals how scientists have battled against the odds for almost a century to detect and decode the neutrino, the smallest and strangest particle of matter in the universe.” The documentary will feature footage and interviews from IceCube and will discuss the experiment’s role in neutrino astronomy.
PBS NOVA will feature IceCube and its science in its “Particles Unknown” documentary premiering on Wednesday, October 6 at 9:00 pm CDT. IceCube will appear near the end of the program, which is also about the hunt for neutrinos, “the universe’s most common—yet most elusive and baffling—particle,” and includes an interview with Hanson, who is also IceCube’s director of operations and the director of WIPAC.
Learn more about IceCube and neutrinos at IceCube’s website.
The IceCube Neutrino Observatory is funded primarily by the National Science Foundation (OPP-1600823 and PHY-1913607) and is headquartered at the Wisconsin IceCube Particle Astrophysics Center, a research center of UW–Madison in the United States. IceCube’s research efforts, including critical contributions to the detector operation, are funded by agencies in Australia, Belgium, Canada, Denmark, Germany, Japan, New Zealand, Republic of Korea, Sweden, Switzerland, the United Kingdom, and the United States. The IceCube EPSCoR Initiative (IEI) also receives additional support through NSF-EPSCoR-2019597.IceCube construction was also funded with significant contributions from the National Fund for Scientific Research (FNRS & FWO) in Belgium; the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG) in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the University of Wisconsin–Madison Research Fund in the U.S.
Balantekin named co-PI on NSF grant to solve cosmic mystery
A team of University of Wisconsin–Madison and New York Institute of Technology physicists has secured a grant from the National Science Foundation (NSF) in an attempt to solve one of science’s greatest mysteries: how the universe formed from stardust.
Many of the universe’s elements, including the calcium found in human bones and iron in skyscrapers, originated from ancient stars. However, scientists have long sought to understand the cosmic processes that formed other elements—those with undetermined origins. Now, UW–Madison professor of physics Baha Balantekin and co-principal investigator Eve Armstrong assistant professor of physics at New York Institute of Technology, will perform the first known research project that uses weather prediction techniques to explain these events. Their revolutionary work will be funded by a two-year $299,998 NSF EAGER grant, an award that supports early-stage exploratory projects on untested but potentially transformative ideas that could be considered “high risk/high payoff.”
While the Big Bang created the first and lightest elements (hydrogen and helium), the next and heavier elements (up to iron on the periodic table) formed later inside ancient, massive stars. When these stars exploded, their matter catapulted into space, seeding that space with elements. Eventually, stardust matter from these supernovae formed the sun and planets, and over billions of years, Earth’s matter coalesced into the first life forms. However, the origins of elements heavier than iron, such as gold and copper, remain unknown. While they may have formed during a supernova explosion, current computational techniques render it difficult to comprehensively study the physics of these events. In addition, supernovae are rare, occurring about once every 50 years, and the only existing data is from the last explosion in 1987.
Large information-rich data sets are obtained from increasingly sophisticated experiments and observations on complicated nonlinear systems. The techniques of Statistical Data Assimilation (SDA) have been developed to handle very nonlinear systems with sparsely sampled data. SDA techniques, akin to the path integral methods commonly used in physics, are used in fields ranging from weather prediction to neurobiology. Armstrong and Balantekin will apply the SDA methods to the vast amount of data accumulated so far in neutrino physics and astrophysics.
With simulated data, in preparation for the next supernova event, the team will use data assimilation to predict whether the supernova environment could have given rise to some heavy elements. If successful, these “forecasts” may allow scientists to determine which elements formed from supernova stardust.
This project will provide an opportunity to the Physics graduate students interested in neutrinos to master an interdisciplinary technique with many other applications.
“Physicists have sought for years to understand how, in seconds, giant stars exploded and created the substances that led to our existence. A technique from another scientific field, meteorology, may help to explain an important piece of this puzzle that traditional tools render difficult to access,” says Armstrong.
The NSF is an independent agency of the U.S. government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health. NSF funding accounts for approximately 27 percent of the total federal budget for basic research conducted at U.S. colleges and universities.
The International Union of Pure and Applied Physics (IUPAP) and the Tata Institute of Fundamental Research (TIFR) in Mumbai, India, have awarded the 2021 Homi Bhabha Medal and Prize to Francis Halzen, the Hilldale and Gregory Breit Distinguished Professor of Physics at the University of Wisconsin–Madison and principal investigator of IceCube, for his “distinguished contributions in the field of high-energy cosmic-ray physics and astroparticle physics over an extended academic career.” Halzen accepted the award at the opening session of the virtual 37th International Cosmic Ray Conference, on July 12, 2021.
The Bhabha Award was established by IUPAP and TIFR in 2010 to honor Dr. Homi Jehangir Bhabha, a cosmic ray physicist well known for the Bhabha-Heitler cascade theory and relativistic positron-electron scattering, also known as Bhabha scattering. Bhabha founded TIFR in 1945 and initiated the nuclear energy program in India in 1951. He initiated experimental programs for the study of cosmic ray particles and their interactions with instruments either carried aloft to the top of the atmosphere with balloons or placed in laboratories at high altitude or deep underground. The Homi Bhabha Medal and Prize consists of a certificate, a medal, a monetary award, and an invitation to visit the TIFR, Mumbai, and the Cosmic Ray Laboratory, Ooty to give public lectures. It is awarded biennially at the International Cosmic Ray Conference.
Born in Belgium, Halzen received his Master’s and PhD degrees from the University of Louvain, Belgium, and has been on the physics faculty at UW–Madison since 1972. The Bhabha Award is just the latest in Halzen’s long and storied career; previous accolades include a 2014 American Ingenuity Award, the 2015 Balzan Prize, a 2018 Bruno Pontecorvo Prize, the 2019 IUPAP Yodh Prize, and the 2021 Bruno Rossi Prize. Halzen is the third IceCube collaborator to win a Bhabha Award after Tom Gaisser in 2015 and Subir Sarkar in 2017.
During his virtual acceptance remarks, Halzen credited his collaborators, saying, “If I made contributions, it is because I ran into incredible collaborators who were leaders in the field, and still are. My ultimate collaborators, of course, I found within the AMANDA collaboration—and now IceCube—who made high-energy neutrinos part of the high-energy cosmic ray spectrum…
“Thanks to everybody, and thanks to IceCube; this prize is shared with all of you.”
Francis Halzen named Vilas Research Professor
UW–Madison physics professor Francis Halzen has been named a Vilas Research Professor. Created “for the advancement of learning,” Vilas Research Professorships are granted to faculty with proven research ability and unusual qualifications and promise. The recipients of the award have contributed significantly to the research mission of the university and are recognized both nationally and internationally.
Halzen, the Gregory Breit and Hilldale Professor of Physics, joined the UW–Madison faculty in 1972. He has made pioneering contributions to particle physics and neutrino astrophysics, and he continues to be the driving force of the international IceCube Collaboration.
Early in his career, Halzen cofounded the internationally recognized phenomenology research institute in the UW–Madison Department of Physics to promote research at the interface of theory and experiment in particle physics. This institute is recognized for this research and for its leadership in the training of postdocs and graduate students in particle physics phenomenology.
The IceCube Neutrino Observatory is the culmination of an idea first conceived in the 1960s, and one in which Halzen has played an integral role in its design, implementation, and data acquisition and analysis for the past three decades. After initial experiments confirmed that the Antarctic ice was ultratransparent and established the observation of atmospheric neutrinos, IceCube was ready to become a reality. From 2004 to 2011, the South Pole observatory was constructed — the largest project ever assigned to a university and one led by Halzen.
After two years of taking data with the full detector, the IceCube Neutrino Observatory opened a new window onto the universe with its discovery of highly energetic neutrinos of extragalactic origin. This discovery heralded the beginning of the exploration of the universe with neutrino telescopes. The IceCube observation of cosmic neutrinos was named the 2013 Physics World Breakthrough of the Year.
Nationally and internationally renowned for this work, Halzen was awarded a 2014 American Ingenuity Award, a 2015 Balzan Prize, a 2018 Bruno Pontecorvo Prize, a 2019 Yodh Prize, and a 2021 Bruno Rossi Prize.
With the Vilas Research Professorship, Halzen is also recognized for his commitment to education and service in the department, university, and international science communities. He has taught everything from physics for nonscience majors to advanced particle physics and special topics courses at UW–Madison. He has actively participated on several departmental and university committees as well as advisory, review, and funding panels. His input is highly sought by committees and agencies that assess future priorities of particle and astroparticle physics research.
“Francis Halzen has had a prolific, internationally recognized research career, has shown excellence as an educator who is able to effectively communicate cutting-edge science on all levels, and has made tireless and valued contributions in service of the department,” says Sridhara Dasu, Physics Department chair. “He is one of the most creative and influential physicists of the last half century and worthy of the prestigious Vilas Research Professorship.”
Vilas awards are supported by the estate of professor, U.S. senator and UW Regent William F. Vilas (1840-1908). The Vilas Research 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.
Halzen joins department colleagues Profs. Vernon Barger and Sau Lan Wu as recipients of this prestigious UW–Madison professorship.
Ke Fang, professor at the University of Wisconsin–Madison, has been selected as the recipient of the 2021 Shakti P. Duggal Award presented by the International Union of Pure and Applied Physics (IUPAP).
The Duggal Award was established after cosmic-ray physicist Shakti Duggal’s untimely death in 1982. In honor of Shakti’s long association with cosmic ray physics and his many contributions to the field during his career, his namesake award is given biennially “to recognize an outstanding young scientist for contributions in any branch of cosmic ray physics.” The first Shakti P. Duggal Award was presented at the 19th International Cosmic Ray Conference at La Jolla in 1985. Previous Duggal Award winners have all achieved recognition and prominence in their careers.
Award winners receive a monetary award and, since 1991, an invitation to visit the Bartol Research Institute of the University of Delaware, where Shakti Duggal worked, to present a colloquium and discuss their work.
Fang’s research focuses on understanding the universe through its energetic messengers, including ultra-high-energy cosmic rays, gamma rays, and high-energy neutrinos. She runs numerical simulations to study theories of astroparticle sources and analyzes data from HAWC, Fermi-LAT, and IceCube. She joined WIPAC and the UW–Madison Physics Department as an assistant professor on January 1, 2021. You can learn more about Fang and her research in this Q&A.
“I am very grateful for this special honor,” said Fang. “As a young researcher, I have received enormous support from my mentors and collaborators, to whom the award truly belongs. I look forward to continuing working on and contributing to cosmic ray physics as a member of the Duggal family.”
Celebrating IceCube’s first decade of discovery
It was the beginning of a grand experiment unlike anything the world had ever seen. Ten years ago today, the IceCube Neutrino Observatory fully opened its eyes for the first time.
Over the course of the previous seven years, dozens of intrepid technicians, engineers, and scientists had traveled to the South Pole—one of the coldest, driest, and most isolated places on Earth—to build the biggest, strangest telescope in the world. Crews drilled 86 holes nearly two-and-a-half kilometers deep and lowered a cable strung with 60 basketball-sized light detectors into each hole. The result was a hexagonal grid of sensors embedded in a cubic kilometer of ice about a mile below the surface of the Antarctic ice sheet. On December 18, 2010, the 5,160th light sensor was deployed in the ice, completing the construction of the IceCube Neutrino Observatory.
The purpose of the unconventional telescope was to detect signals from passing astrophysical neutrinos: mysterious, tiny, extremely lightweight particles created by some of the most energetic and distant phenomena in the cosmos. IceCube’s founders believed that studying these astrophysical neutrinos would reveal hidden parts of the universe. Over the course of the next decade, they would be proven right.
IceCube began full operations on May 13, 2011 — ten years ago today — when the detector took its first set of data as a completed instrument. Since then, IceCube has been watching the cosmos and collecting data continuously for a decade.
During its first few years of operation, IceCube accumulated vast amounts of data, but it wasn’t until 2013 that the observatory yielded its first major results.