Research, teaching and outreach in Physics at UW–Madison
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.
Study led by UW–Madison researcher confirms star wreck as source of extreme cosmic particles
Astronomers have long sought the launch sites for some of the highest energy protons in our galaxy. Now, a study using 12 years of data from NASA’s Fermi Gamma-ray Space Telescope (Fermi) confirms that a remnant of a supernova, or star explosion, is just such a place, solving a decade-long cosmic mystery.
Previously, Fermi has shown that the shock waves of exploded stars boost particles to speeds comparable to that of light. Called cosmic rays, these particles mostly take the form of protons, but can include atomic nuclei and electrons. Because they all carry an electric charge, their paths become scrambled as they whisk through our galaxy’s magnetic field, which masks their origins. But when these particles collide with interstellar gas near the supernova remnant (SNR), they produce a telltale glow in gamma rays—the highest-energy light there is.
“Theorists think the highest energy cosmic ray protons in the Milky Way reach a million billion electron volts, or PeV energies,” said Ke Fang, an assistant professor of physics at the Wisconsin IceCube Particle Astrophysics Center (WIPAC), a research center at the University of Wisconsin–Madison. “The precise nature of their sources, which we call PeVatrons, has been difficult to pin down.”
Fang, who led the study, performed the data analysis and developed the theory models. The research team identified a few suspected PeVatrons, including one at the center of our galaxy. Naturally, SNR top the list of candidates. Yet out of about 300 known remnants, only a few have been found to emit gamma rays with sufficiently high energies.
Design and performance of the prototype Schwarzschild-Couder telescope camera
The debut of a new detector has many “firsts”: the first assembly, the first shift, the first light, the first detection… But if there’s one thing that makes a debut official—sort of like a detector’s birth certificate—it’s the detailed description of how the detector was built and how it performs.
And this is achieved in a new paper by members of the Cherenkov Telescope Array Consortium, published in the Journal of Astronomical Telescopes, Instruments and Systems. The paper documents the design of the camera of the prototype Schwarzschild-Couder Telescope (pSCT), a medium-sized candidate telescope for the Cherenkov Telescope Array (CTA). The paper also includes performance metrics that show its potential as a very-high-energy gamma-ray detector and that have already been used to plan an upgrade, a project which is now well underway. Very high energy gamma rays are the highest energy photons in the universe and can unveil the physics of extreme objects, including black holes and possibly dark matter.
The pSCT uses novel dual-mirror optics, rather than more traditional single-mirror optics, and relies on high-speed electronics to cover CTA’s middle energy range from 80 GeV to 50TeV. This camera was developed by a team spanning multiple universities and co-led by UW–Madison physics professor Justin Vandenbroucke, who has been working on this project since 2009.
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.
Highest-energy Cosmic Rays Detected in Star Clusters
For decades, researchers assumed the cosmic rays that regularly bombard Earth from the far reaches of the galaxy are born when stars go supernova — when they grow too massive to support the fusion occurring at their cores and explode.
Those gigantic explosions do indeed propel atomic particles at the speed of light great distances. However, new research suggests even supernovae — capable of devouring entire solar systems — are not strong enough to imbue particles with the sustained energies needed to reach petaelectronvolts (PeVs), the amount of kinetic energy attained by very high-energy cosmic rays.
And yet cosmic rays have been observed striking Earth’s atmosphere at exactly those velocities, their passage marked, for example, by the detection tanks at the High-Altitude Water Cherenkov (HAWC) observatory near Puebla, Mexico. Instead of supernovae, the researchers — including UW–Madison’s Ke Fang — posit that star clusters like the Cygnus Cocoon serve as PeVatrons — PeV accelerators — capable of moving particles across the galaxy at such high energy rates.
IceCube detection of a high-energy particle proves 60-year-old theory
On Dec. 8, 2016, a high-energy particle hurtled to Earth from outer space at close to the speed of light. The particle, an electron antineutrino, smashed into an electron deep inside the ice sheet at the South Pole. This collision produced a particle that quickly decayed into a shower of secondary particles, triggering the sensors of the IceCube Neutrino Observatory, a massive telescope buried in the Antarctic glacier.
IceCube had seen a Glashow resonance event, a phenomenon predicted by Nobel laureate physicist Sheldon Glashow in 1960. With this detection, scientists provided another confirmation of the Standard Model of particle physics. It also further demonstrated the ability of IceCube, which detects nearly massless particles called neutrinos using thousands of sensors embedded in the Antarctic ice, to do fundamental physics. The result was published March 10 in Nature.
When you think of scientific meccas throughout the world, Madison, Wisconsin might not be the first place that comes to mind. But for astroparticle physicist Ke Fang, Madison is the place to be. That’s because it’s home to the Wisconsin IceCube Particle Astrophysics Center (WIPAC): the “leader of particle astrophysics in the world,” according to Fang. “Throughout the years, there have been all kinds of meetings and workshops that drive people in this field to Madison because it’s the center for particle astrophysics,” she says.
Originally from Huangshan, China, Fang earned a B.S. in physics from the University of Science and Technology of China. Afterward, Fang moved to the United States for graduate school and earned her PhD in astrophysics from the University of Chicago in 2015. Following that, she went to the University of Maryland and the Goddard Space Flight Center for a Joint Space-Science Institute fellowship. Most recently, Fang was a NASA Einstein Fellow at Stanford University in California.
Now, Fang has joined WIPAC and the UW–Madison Physics Department as an assistant professor. To welcome Fang and learn more about her, we met up on—where else?—Zoom for an interview.
Can you summarize your research?
I use both experiments and theory to understand extreme activities of our universe. We receive multiple types of messengers from the universe—all the way from optical light to gamma rays, cosmic rays, neutrinos, and gravitational waves. These messengers can be emitted by a common source, such as a binary neutron star merger. Specifically, I use theoretical models to understand how these astrophysical events produce different messengers, whether theoretical models explain the data, and how the data compare with theoretical models. I also use the HAWC Observatory, the IceCube Neutrino Observatory, and the Fermi Large Area Telescope (Fermi-LAT) to observe or to find sources directly. For example, I jointly analyze the Fermi-LAT and HAWC data to observe gamma-ray sources from 0.1 GeV to 100 TeV—across six orders of magnitude. Studies using multiple messengers and wavelengths are rewarding because they help us get a full picture of what these astrophysical sources look like.
How did you get into your field of research?
When I started graduate school, high-energy astrophysics was rather new; it’s a field that has quickly grown in the past decade or so. High-energy astrophysics traditionally refers to astrophysics with X-ray observations, because X-rays are higher in energy compared to the optical band that astronomers traditionally use. But in the last few years, high-energy astrophysics has had another burst of delving into even higher energies. When we move up in energy, by millions or billions, we see many new sources that were previously not observable in the X-ray band, or different aspects of sources that were previously seen at lower energies. And there are so many unknowns in this field; we can see surprising things at the highest energies, and many of those observations are discoveries. I think that’s really intriguing.
What attracted you to UW–Madison and WIPAC?
I think it’s pretty fair to say that WIPAC—with IceCube, CTA, HAWC, Fermi-LAT, ARA, and IceCube-Gen2—is now the leader of particle astrophysics in the world. I think there’s a close match between my expertise and what is currently being done at WIPAC, and I’m excited about joining the department and joining these explorations of higher and higher energy neutrinos and gamma rays.
What’s one thing you hope students who take a class with you will come away with?
The content you learn from a class is limited, but the contexts where you could apply the knowledge are unlimited.
What is your favorite particle?
Neutrino. If I have to pick one, neutrino is the one that I have in my heart.
What hobbies/other interests do you have?
Cooking! I like to explore different things. I come from China, so Chinese cuisine is what I started from when I just moved to the United States. But after all these years, I’m getting more exposed to different types of cuisines and starting to explore more, like with Thai and Italian. When I go to nice restaurants, I try to remember the name of the dish and find the recipe online.
Welcome, Professor Lu Lu!
New UW–Madison assistant professor of physics Lu Lu’s research program combines the past with the future. Her research looks for sources of ultrahigh energy particles, which is done by analyzing data that has already been collected. As she says, “Maybe data is already talking to us, we just haven’t looked.” But she is also working toward improving future data collection, which will require more technologically-advanced detectors. “My teachers, my great masters, have taught me that the current young generation has the responsibility to look into new techniques to go to the future for younger generations to proceed forward,” she says about her work in sensor R&D.
On January 1, Professor Lu joined the Department of Physics and IceCube. Most recently, she was a postdoctoral fellow at the International Center for Hadron Astrophysics at Chiba University in Japan. To welcome her, we sat down for a (virtual) interview.
What are your research interests?
My prime interest is astroparticle physics, and my ultimate goal is to find the sources of the highest energy particles in the universe. These particles carry energy of about 1020 electronvolts. This is higher energy than what we have from the Large Hadron Collider and human technologies. The real attractiveness here is we don’t know how nature accelerates these particles. And once we identify the sources, we can test new theories beyond the Standard Model using sources crated by nature.
What are one or two main projects you focus your research on?
I’m involved in two experiments. One is IceCube, the other is Pierre Auger Observatory. I was doing cosmic ray analysis, but cosmic rays are usually charged particles and they are deflected in the magnetic field of the galaxy; they would not travel in a straight line. IceCube studies neutrinos which are neutral particles, they travel directly from the source. Pierre Auger detects ultrahigh energy photons, which are also neutral particles. One thing I want to do immediately after I join Madison is to combine these two experiments to do a joint analysis. We have photon candidates but we haven’t really tried to connect them in the multimessenger regime. By combining Pierre Auger photons with IceCube neutrinos, we could possibly find a transient source, a source that doesn’t constantly emit ultrahigh energy photons or neutrinos but all of a sudden there’s a flare. This type of analysis has never been done, but we have data on disks.
The second thing I’m interested in is using new sensor technologies. In IceCube, we have Gen2 being planned right now. Instead of using a single photon sensor, we’d use a more sensitive design and R&D. UW–Madison is taking the lead of designing this future detector. There’s also radio technology. So, to detect the highest energy neutrinos we need to build a large instrument volume. With optical array, it is really hard to scale up because one has to drill holes inside the South Pole, which is really expensive. But radio technology doesn’t have to go so deep, so they can bury their detectors on the surface areas, and the radiowaves can transmit further away than the optical photons in ice. For optical you have to make the detectors very dense, but for radio you can make the antennas further apart, so that means you can have a larger area and detect more events easily. I think radio is the way to go for the future.
You said you have a lot of data collected already and just need to analyze it. How do you analyze the data from these detectors?
We would have to search for photon candidates from the data from Auger, and identify where it comes from and what the time this event happened. Correspondingly, do we see neutrinos from IceCube coming from the same direction and at the same time? Because you can never be sure it’s a photon. It could be a proton. We then want to build a statistical framework to combine different multimessengers together in real time.
What does it mean if you find a photon in coincidence with a neutrino?
Cosmic rays were first detected more than 100 years ago, and there’s a rich history of studying where they come from. The mystery of origins still remains today because our poor knowledge on the galactic/extragalactic magnetic fields and mass composition of cosmic rays. In my opinion, the most probable way to solve this puzzle is to use neutral particles. If we can identify ultrahigh energy photons in coincidence with neutrinos, that is a smoking gun that we are actually looking at a source and we can finally pin down where in the universe is accelerating high energy particles. And therefore, we can study particle physics maybe beyond Standard Model. It’s just like a lab created by the universe to test particle physics.
What is your favorite element and/or elementary particle?
My favorite elementary particle is the electron anti-neutrino. I like muons, too. My favorite element is hydrogen.
What hobbies and interests do you have?
I’m afraid I’ll disappoint you because my hobby is related to my research: Augmented reality. When I heard about something called Microsoft Hololens, I thought, I could make IceCube a hologram. I bought these special glasses, and then made a program on it and used it for some outreach events. But the glasses are very expensive, so people said, “Okay we can’t buy hologram glasses.” So I moved it to mobile phones so that everyone could look at it for fun. It’s called IceCubeAR (note: download it for iPhones or Android phones). I made it with a group of friends in Tokyo.