IceCube Observes Seven Astrophysical Tau Neutrino Candidates

Neutrinos are tiny, weakly interacting subatomic particles that can travel astronomical distances undisturbed. As such, they can be traced back to their sources, revealing the mysteries surrounding the cosmos. High-energy neutrinos that originate from the farthest reaches beyond our galaxy are called astrophysical neutrinos and are the main subject of study for the IceCube Neutrino Observatory, a cubic-kilometer-sized neutrino telescope at the South Pole. In 2013, IceCube presented its first evidence of high-energy astrophysical neutrinos originating from cosmic accelerators, beginning a new era in astronomy. 

These cosmic messengers come in three different flavors: electron, muon, and tau, with astrophysical tau neutrinos being exceptionally difficult to pin down. Now, in a new study recently accepted as an “Editors’ Suggestion” by Physical Review Letters, the IceCube Collaboration presents the discovery of the once-elusive astrophysical tau neutrinos, a new kind of astrophysical messenger. 

IceCube detects neutrinos using cables (strings) of digital optical modules (DOMs), with a total of 5,160 DOMs embedded deep within the Antarctic ice. When neutrinos interact with molecules in the ice, charged particles are produced that then emit blue light while traveling through the ice, which is then registered and digitized by the individual DOMs. The light produces distinctive patterns, one of which is double cascade events from high-energy tau neutrino interactions within the detector.

The production of a double pulse waveform. The photons from a neutrino interaction (blue) arrive at the top middle DOM at time tI, producing the first peak in the waveform, while photons from the tau lepton decay (purple) arrive at the same DOM at time tD, producing the second peak. Credit: Jack Pairin/IceCube CollaborationThe production of a double pulse waveform. The photons from a neutrino interaction (blue) arrive at the top middle DOM at time tI, producing the first peak in the waveform, while photons from the tau lepton decay (purple) arrive at the same DOM at time tD, producing the second peak. Credit: Jack Pairin/IceCube Collaboration

Since prior IceCube analyses saw hints from searches for subtle signatures produced by astrophysical tau neutrinos, the researchers remained motivated to pinpoint tau neutrinos. After rendering each event into three images (see figure below), they trained convolutional neural networks (CNNs) optimized for image classification to distinguish images produced by tau neutrinos from images produced by various backgrounds. After having simulations run that confirmed its sensitivity to tau neutrinos, the technique was then applied to 10 years of IceCube data acquired between 2011 and 2020. The result was seven strong candidate tau neutrino events. 

“The detection of seven candidate tau neutrino events in the data, combined with the very low amount of expected background, allows us to claim that it is highly unlikely that backgrounds are conspiring to produce seven tau neutrino imposters,” said Doug Cowen, a professor of physics at Penn State University and one of the study leads. “The discovery of astrophysical tau neutrinos also provides a strong confirmation of IceCube’s earlier discovery of the diffuse astrophysical neutrino flux.”

Candidate astrophysical tau neutrino detected on November 13, 2019. Each column corresponds to one of the three neighboring strings of the selected event. Each figure in the top row shows the DOM number, proportional to the depth, versus the time of the digitized PMT signal in 3-ns bins, with the bin color corresponding to the size of the signal in each time bin, for each of the three strings. The total number of photons detected by each string is provided at the upper left in each figure. In the most-illuminated string (left column), the arrival of light from two cascades is visible as two distinct hyperbolas. The bottom row of figures shows the “saliency” for one of the CNNs for each of the three strings. The saliency shows where changes in light level have the greatest impact on the value of the CNN score. The black line superimposed on the saliency plots shows where the light level goes to zero and is effectively an outline of the figures in the top row. The saliency is largest at the leading and trailing edges of the light emitted by the two tau neutrino cascades, showing that the CNN is mainly sensitive to the overall structure of the event. Credit: IceCube CollaborationCandidate astrophysical tau neutrino detected on November 13, 2019. Each column corresponds to one of the three neighboring strings of the selected event. Each figure in the top row shows the DOM number, proportional to the depth, versus the time of the digitized PMT signal in 3-ns bins, with the bin color corresponding to the size of the signal in each time bin, for each of the three strings. The total number of photons detected by each string is provided at the upper left in each figure. In the most-illuminated string (left column), the arrival of light from two cascades is visible as two distinct hyperbolas. The bottom row of figures shows the “saliency” for one of the CNNs for each of the three strings. The saliency shows where changes in light level have the greatest impact on the value of the CNN score. The black line superimposed on the saliency plots shows where the light level goes to zero and is effectively an outline of the figures in the top row. The saliency is largest at the leading and trailing edges of the light emitted by the two tau neutrino cascades, showing that the CNN is mainly sensitive to the overall structure of the event. Credit: IceCube CollaborationCowen added that the probability of the background mimicking the signal was estimated to be less than one in 3.5 million. 

UMD Research Scientist Erik Blaufuss served as an internal reviewer for the analysis, carefully studying the methods and techniques used to make the discovery. Assistant Professor Brian Clark leads the scientific working group in IceCube that produced the result. The IceCube collaboration includes several UMD faculty, including  Kara Hoffman, Greg Sullivan, and Michael Larson, in addition to several graduate students and postdocs. The UMD group plays a leading role in the maintenance and operations of the detector, as well as the simulation and analysis of the data. 

Future analyses will incorporate more of IceCube’s strings, since this study used just three of them. The new analysis would increase the sample of tau neutrinos that can then be used to perform the first three-flavor study of neutrino oscillations—the phenomenon where neutrinos change flavors—over cosmological distances. This type of study could address questions such as the mechanism of neutrino production from astrophysical sources and the properties of space through which neutrinos travel. 

Currently, there is no tool specifically designed to determine the energy and direction of tau neutrinos that produce the signatures seen in this analysis. Such an algorithm could be used to better differentiate a potential tau neutrino signal from background and to help identify candidate tau neutrinos in real time at the South Pole. Similar to current IceCube real-time alerts issued for other neutrino types, alerts for tau neutrinos could be issued to the astronomical community for follow-up studies.

All in all, this exciting discovery comes with the “intriguing possibility of leveraging tau neutrinos to uncover new physics,” said Cowen. 

+ info “Observation of Seven Astrophysical Tau Neutrino Candidates with IceCube,” The IceCube Collaboration: R. Abbasi et al. Accepted by Physical Review Letters. arxiv.org/abs/2403.02516

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Ana Maria Rey to Speak at Graduate Commencement Ceremony

For Ana Maria Rey (Ph.D. ’04, physics), the path to a highly successful career as a theoretical physicist and researcher began more than three decades ago in her home country of Colombia, with an inspiring high school physics teacher, the brilliance of Isaac Newton and her own boundless curiosity.

Ana Maria Rey.  Courtesy of same.Ana Maria Rey. Courtesy of same. Ana Maria Rey. Photo courtesy of same. Click image to download hi-res version.

I had a physics teacher in high school, and he was amazing, he taught me about Newton’s laws of motion,” Rey recalled. “I was so excited that I could write an equation and predict the behavior of objects that I kept asking him to give me books so I could keep working and solve more problems because it was all so interesting to me.”

A few years later, after Rey earned her bachelor’s degree in physics in Colombia and began her Ph.D. in physics at the University of Maryland, her future as a scientist began to come into sharper focus. Thanks to a game-changing connection with Distinguished University Professor of Physics and Nobel laureate William Phillips, Rey charted a course toward breakthrough research in atomic and molecular physics and laid the groundwork for a fruitful collaboration with the National Institute of Standards and Technology (NIST), one that is still going strong today.

“For me, NIST and the University of Maryland are just blended. I can’t separate them because they’re so connected,” Rey explained. “UMD gave me a strong foundation, all my research experience, the collaboration experience, everything I learned about how I should talk to experimentalists was thanks to the University of Maryland/NIST partnership. My research was going on at NIST but this only could happen because I was at UMD, so I think when I decided to go to the University of Maryland it was one of the best decisions I have ever made.”

Currently an adjunct professor at the University of Colorado Boulder, Rey has been a NIST Fellow since 2017 and a Fellow of JILA, the joint physics institute of CU Boulder and NIST, since 2012. She has earned a host of prestigious awards including a MacArthur Fellowship and the 2014 Presidential Early Career Award for Scientists and Engineers. In 2019, Rey became the first Hispanic woman to win the Blavatnik National Award for Young Scientists, and in 2023 she was elected to the National Academy of Sciences, one of the highest professional honors for a scientist.   

A big, big honor

In May 2024, nearly 20 years after earning her Ph.D., Rey will return to Maryland—a place that is still very close to her heart—to deliver the keynote speech at the Graduate Commencement Ceremony for UMD’s College of Computer, Mathematical, and Natural Sciences. She’s honored and humbled by the invitation.

“It’s a big, big honor. For me it was really special to feel that a university that has been so important in my career, to determining who I am, has asked me to give a speech like this,” Rey said. “I feel like inspiring students is one of my biggest roles and that’s why I find so touching the possibility to give the commencement speech because maybe this is a way I can tell them how I feel and try to encourage them to make the most of their future.”

A Nobel Prize-winning inspiration

When Rey began her graduate work at UMD she planned to pursue research in plasma physics—that is, until she attended an inspiring lecture by Phillips about his pioneering work with atoms and lasers.

“I heard Bill’s talk about how he was cooling atoms with light, and I found it fascinating. He was fantastic.  I approached my plasma physics advisor Adil Hasam afterward and I told him, ‘I feel that this is the direction that I want to pursue’ and he was totally supportive,” Rey recalled. “He encouraged me to reach out to Charles Clark, who at that time was the chief of the electron optical physics division at NIST and that is what I did.  Charles was very welcoming and told me that I could start working on ultra-cold atoms trapped in periodic potentials using lasers. That is how my Ph.D. adventure started.”

From then on, Rey’s research efforts really took off. At UMD and NIST and later at JILA, her work has focused on atomic, molecular and optical physics as well as condensed matter physics and quantum information science, setting the stage for one of her proudest achievements—her contribution to the most accurate atomic clock ever created.

“My goal is to try to understand the deepest secrets of the universe and try to use them for something useful,” Rey explained. “Understanding the collisions with these atoms has allowed us to create a clock that is really one of the best timekeepers that we have ever been able to construct, and we can now predict that they offer many other, unique possibilities.”

“You need to be excited”

A leading researcher in the Quantum Systems Accelerator, Rey has published more than 200 papers. And, after more than two decades of research, she’s still as motivated and excited about her work as she was the day it all began.

“As a scientist, you have to work a lot to make progress, so you absolutely need to be excited,” Rey explained. “I love it. Every day I’m surrounded by so many exciting experiments and so many things that I need to learn and understand. And every time that I learn something new, it really makes my day.”

Rey hopes she can share that excitement when she speaks to students and their families at the CMNS Graduate Commencement Ceremony in May. Her goal is to inspire the next generation of scientists to create success stories of their own.

“I would like to serve as a role model the way others have done for me,” Rey said. “If I am able to inspire new generations to become physicists, to advance science and do better, that’s one of my great ambitions, and it would be a great honor to feel that I’m doing that. So, my message to them is you have now in your hands the possibility to make a change in the world, so use all the knowledge that you’ve acquired to make that happen.“

 

A Focused Approach Can Help Untangle Messy Quantum Scrambling Problems

The world is a cluttered, noisy place, and the ability to effectively focus is a valuable skill. For example, at a bustling party, the clatter of cutlery, the conversations, the music, the scratching of your shirt tag and almost everything else must fade into the background for you to focus on finding familiar faces or giving the person next to you your undivided attention. 

Similarly, nature and experiments are full of distractions and negligible interactions, so scientists need to deliberately focus their attention on sources of useful information. For instance, the temperature of the crowded party is the result of the energy carried by every molecule in the air, the air currents, the molecules in the air picking up heat as they bounce off the guests and numerous other interactions. But if you just want to measure how warm the room is, you are better off using a thermometer that will give you the average temperature of nearby particles rather than trying to detect and track everything happening from the atomic level on up. A few well-chosen features—like temperature and pressure—are often the key to making sense of a complex phenomenon.

It is especially valuable for researchers to focus their attention when working on quantum physics. Scientists have shown that quantum mechanics accurately describes small particles and their interactions, but the details often become overwhelming when researchers consider many interacting quantum particles. Applying the rules of quantum physics to just a few dozen particles is often more than any physicist—even using a supercomputer—can keep track of. So, in quantum research, scientists frequently need to identify essential features and determine how to use them to extract practical insights without being buried in an avalanche of details.A collection of quantum particles can store information in various collective quantum states. The above model represents the states as blue nodes and illustrates how interactions can scramble the organized information of initial states into a messy combination by mixing the options along the illustrated links. (Credit: Amit Vikram, UMD)A collection of quantum particles can store information in various collective quantum states. The above model represents the states as blue nodes and illustrates how interactions can scramble the organized information of initial states into a messy combination by mixing the options along the illustrated links. (Credit: Amit Vikram, UMD)

In a paper published in the journal Physical Review Letters in January 2024, Professor Victor Galitski and JQI graduate student Amit Vikram identified a new way that researchers can obtain useful insights into the way information associated with a configuration of particles gets dispersed and effectively lost over time. Their technique focuses on a single feature that describes how various amounts of energy can be held by different configurations a quantum system. The approach provides insight into how a collection of quantum particles can evolve without the researchers having to grapple with the intricacies of the interactions that make the system change over time.

This result grew out of a previous project where the pair proposed a definition of chaos for the quantum world. In that project, the pair worked with an equation describing the energy-time uncertainty relationship—the less popular cousin of the Heisenberg uncertainty principle for position and momentum. The Heisenberg uncertainty principle means there’s always a tradeoff between how accurately you can simultaneously know a quantum particle’s position and momentum. The tradeoff described by the energy-time uncertainty relationship is not as neatly defined as its cousin, so researchers must tailor its application to different contexts and be careful how they interpret it. But in general, the relationship means that knowing the energy of a quantum state more precisely increases how long it tends to take the state to shift to a new state.

When Galitski and Vikram were contemplating the energy-time uncertainty relationship they realized it naturally lent itself to studying changes in quantum systems—even those with many particles—without getting bogged down in too many details. Using the relationship, the pair developed an approach that uses just a single feature of a system to calculate how quickly the information contained in an initial collection of quantum particles can mix and diffuse.

The feature they built their method around is called the spectral form factor. It describes the energies that quantum physics allows a system to hold and how common they are—like a map that shows which energies are common and which are rare for a particular quantum system.

The contours of the map are the result of a defining feature of quantum physics—the fact that quantum particles can only be found in certain states with distinct—quantized—energies. And when quantum particles interact, the energy of the whole combination is also limited to certain discrete options. For most quantum systems, some of the allowed energies are only possible for a single combination of the particles, while other energies can result from many different combinations. The availability of the various energy configurations in a system profoundly shapes the resulting physics, making the spectral form factor a valuable tool for researchers.

Galitski and Vikram tailored a formulation of the energy time uncertainty relationship around the spectral form factor to develop their method. The approach naturally applies to the spread of information since information and energy are closely related in quantum physics. 

While studying this diffusion, Galitski and Vikram focused their attention on an open question in physics called the fast-scrambling conjecture, which aims to pin down how long it takes for the organization of an initial collection of particles to be scrambled—to have its information mixed and spread out among all interacting particles until it becomes effectively unrecoverable. The conjecture is not concerned just with the fastest scrambling that is possible for a single case, but instead, it is about how the time that the scrambling takes changes based on the size or complexity of the system. 

Information loss during quantum scrambling is similar to an ice sculpture melting. Suppose a sculptor spelled out the word “swan” in ice and then absentmindedly left it sitting in a tub of water on a sunny day. Initially, you can read the word at a glance. Later, the “s” has dropped onto its side and the top of the “a” has fallen off, making it look like a “u,” but you can still accurately guess what it once spelled. But, at some point, there’s just a puddle of water. It might still be cold, suggesting there was ice recently, but there’s no practical hope of figuring out if the ice was a lifelike swan sculpture, carved into the word “swan” or just a boring block of ice. 

How long the process takes depends on both the ice and the surroundings: Perhaps minutes for a small ice cube in a lake or an entire afternoon for a two-foot-tall centerpiece in a small puddle.

The ice sculpture is like the initial information contained in a portion of the quantum particles, and the surrounding water is all the other quantum particles they can interact with. But, unlike ice, each particle in the quantum world can simultaneously inhabit multiple states, called a quantum superposition, and can become inextricably linked together through quantum entanglement, which makes deducing the original state extra difficult after it has had the chance to change. 

For practical reasons, Galitski and Vikram designed their technique so that it applies to situations where researchers never know the exact states of all the interacting quantum particles. Their approach works for a range of cases spanning those where information is stored in a small chunk of all the interacting quantum particles to ones where the information is on a majority of particles—anything from an ice cube in a lake to a sculpture in a puddle. This gives the technique an advantage over previous approaches that only work for information stored on a few of the original particles.

Using the new technique, the pair can get insight into how long it takes a quantum message to effectively melt away for a wide variety of quantum situations. As long as they know the spectral form factor, they don’t need to know anything else. 

“It's always nice to be able to formulate statements that assume as little as possible, which means they're as general as possible within your basic assumptions,” says Vikram, who is the first author of the paper. “The neat little bonus right now is that the spectral form factor is a quantity that we can in principle measure.”

The ability of researchers to measure the spectral form factor will allow them to use the technique even when many details of the system are a mystery. If scientists don’t have enough details to mathematically derive the spectral form factor or to tailor a custom description of the particles and their interactions, a measured spectral form factor can still provide valuable insights. 

As an example of applying the technique, Galitski and Vikram looked at a quantum model of scrambling called the Sachdev-Ye-Kitaev (SYK) model. Some researchers believe there might be similarities between the SYK model and the way information is scrambled and lost when it falls into a black hole. 

Galitski and Vikram’s results revealed that the scrambling time became increasingly long as they looked at larger and larger numbers of particles instead of settling into conditions that scrambled as rapidly as possible. 

“Large collections of particles take a really long time to lose information into the rest of the system,” Vikram says. “That is something we can get in a very simple way without knowing anything about the structure of the SYK model, other than its energy spectrum. And it's related to things people have been thinking about simplified models for black holes. But the real inside of a black hole may turn out to be something completely different that no one's imagined.”

Galitski and Vikram are hoping future experiments will confirm their results, and they plan to continue looking for more ways to relate a general quantum feature to the resulting dynamics without relying on many specific details. They and their colleagues are also investigating properties of the spectral form factor that every system should satisfy and are working to identify constraints on scrambling that are universal for all quantum systems.

Original story by Bailey Bedford: https://jqi.umd.edu/news/focused-approach-can-help-untangle-messy-quantum-scrambling-problems

This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0001911. 

Researchers Win UMD Quantum Invention of the Year Award

Since 1987, the University of Maryland has presented an annual Invention of the Year Award to celebrate all the innovative work produced by researchers on the campus. This year JQI researchers and their colleagues have won in the quantum category for a new method for counting particles of light—photons—without destroying them. Non-destructively counting photons has potential uses in quantum computers and quantum networks that store information in quantum states of light.

The co-inventors nominated for their non-destructive photon counting protocol are: 

In 2023, researchers at UMD disclosed a total of 154 inventions, and three teams of inventors were selected as finalists for the award in each of the four categories of Information Sciences, Life Sciences, Physical Sciences and Quantum. The inventions were judged based on their technical merit, the improvements they offer, and their commercial potential and overall benefit to society. 

“It's really nice to be recognized, especially for quantum science because I think sometimes we feel as theorists that we're a little bit more distant from scientific applications, in the sense that a lot of times we're not working on things that are instantly profitable,” says Fechisin. “So it's nice for people to recognize that there's still interesting work and exciting work being done.”

Fechisin is the first author of a paper that the group has posted to the arXiv preprint server that describes the procedure they invented. The team’s approach uses an organized sheet of atoms to absorb photons temporarily and serve as an intermediary. Probing the atoms allows the number of photons that were absorbed to be measured before they are eventually emitted back out of the atoms. 

“Photons are hard to work with and don't typically interact with one another,” Fechisin says. “But atoms are easy to work with, and you can make them interact. So you take information stored in photons, which don't easily talk to each other and are hard to pin down, and you convert the photonic data into atomic data.”

In the paper, the team described the procedure and outlined what is needed from the atoms so that they and the photons can be quantum mechanically tied together and measured, without destroying the photons.

The approach works because when atoms in the array absorb light, they can be made to cycle between quantum states at different rates that depend on how many photons have been absorbed. In their proposal, the team described how a series of measurements can home in on the particular frequency the atoms are cycling at, identifying the corresponding number of photons. After the count is obtained, the atoms can emit the photons. The paper also includes an analysis of how to choose an efficient set of measurements to reliably identify the correct cycling frequency.

“The protocol works in a way that's kind of elegant,” Fechisin says. “You have information stored in these unwieldy photons, you turn it into atomic excitations over which you have a much greater degree of control, and then you just very neatly get this signal, which contains exactly the information that you want.”

The team plans to publish their proposal in a peer-reviewed journal and hopes that other research groups will apply this invention in their future experiments.

Original story by Bailey Bedford: https://jqi.umd.edu/news/jqi-researchers-win-2023-umd-quantum-invention-year-award