New IceCube detection proves 60-year-old theory
On December 6, 2016, a high-energy particle called an electron antineutrino was hurtling through space at nearly the speed of light. Normally, the ghostly particle would zip right through the Earth as if it weren't even there.
But this particle just so happened to smash into an electron deep inside the South Pole's glacial ice. The collision created a new particle, known as the W- boson. That boson quickly decayed, creating a shower of secondary particles.
The whole thing played out in front of the watchful detectors of a massive telescope buried in the Antarctic ice, the IceCube Neutrino Observatory. This enabled IceCube to make the first ever detection of a Glashow resonance event, a phenomenon predicted 60 years ago by Nobel laureate physicist Sheldon Glashow.
This detection provides the latest confirmation of the Standard Model, the name of the particle physics theory explaining the universe's fundamental forces and particles.
"Finding it wasn't necessarily a surprise, but that doesn't mean I wasn't very happy to see it," said Claudio Kopper, an associate professor in Michigan State University's Department of Physics and Astronomy in the College of Natural Science. Kopper and his departmental colleague, assistant professor Nathan Whitehorn, lead IceCube's Diffuse and Atmospheric Flux Working Group behind the discovery.
The international IceCube Collaboration published this result online on March 11 in the journal Nature.
"Even three years ago, I didn't think IceCube would be able to make this measurement, or at least as well as we did," Whitehorn said.
A 3-D plot with columns of green, blue, yellow and orange spheres and other round shapes give a visual representation of the Glashow resonance event detection.
This detection further demonstrates the ability of IceCube, which observes nearly massless particles called neutrinos using thousands of sensors embedded in the Antarctic ice, to do fundamental physics.
Although the Spartans lead the working group, they emphasized that this discovery was a team effort, powered by the paper's three lead analysts: Lu Lu, an assistant professor at University of Wisconsin-Madison; Tianlu Yuan, an assistant scientist at the Wisconsin IceCube Particle Astrophysics Center, or WIPAC; and Christian Haack, a postdoc at the Technical University of Munich.
"We lead weekly meetings, we talk about how the work is done, we ask hard questions," said Kopper. "But without the people doing the actual analysis, we wouldn't have anything."
"Our job is to be the doubters-in-chief," Whitehorn said. "The lead authors did a great job convincing everyone that this event was a Glashow resonance."
The particle physics community has been anticipating such a detection, but Glashow resonance events are extremely rare by nature and technologically challenging to detect.
"When Glashow was a postdoc at Niels Bohr, he could never have imagined that his unconventional proposal for producing the W- boson would be realized by an antineutrino from a faraway galaxy crashing into Antarctic ice," said Francis Halzen, professor of physics at the University of Wisconsin-Madison, the headquarters of IceCube maintenance and operations, and principal investigator of IceCube.
A Glashow resonance event requires an electron antineutrino with a cosmic amount of energy -- at least 6.3 peta-electronvolts, or PeV. For comparison, that's about 1,000 times more energy than that of the most energetic particles produced by the Earth's most powerful particle accelerators.
Since IceCube started fully operating in 2011, it has detected hundreds of high-energy neutrinos from space. Yet the neutrino in December 2016 was only the third with an energy higher than 5 PeV.
And simply having a high-energy neutrino is not sufficient to detect a Glashow resonance event. The neutrino then has to interact with matter, which is not a guarantee. But IceCube encompasses quite a bit of matter in the form of Antarctic ice.
The IceCube Laboratory, lighted red against the night sky, is small in this landscape photograph of the South Pole white tundra, which also captures yellow stars and light green auroras.
The observatory's detector array has been built into the ice, spanning nearly 250 acres with sensors reaching up to about a mile deep. All told, IceCube boasts a cubic kilometer of coverage, watching over a billion metric tons of extremely clear ice.
That's what it takes to detect neutrinos, along with a team of scientists who have the skill and determination to spot rare events.
IceCube's more than 5,000 detectors take in a tremendous firehose of light, Whitehorn said. Detecting the Glashow resonance meant researchers had to pick out a handful of telltale photons, individual particles of light, from that firehose spray.
"This is some of the most impressive technical work I've ever seen," Whitehorn said, calling the team unstoppable over the years-long effort to confirm this was a Glashow resonance event.
Making the work even more impressive was the fact that the lead authors -- Lu, Yuan and Haack -- were in three countries on three different continents during the analysis. Lu was a postdoc at Chiba University in Japan, Yuan was at WIPAC in the U.S. and Haack was a doctoral student at Rheinisch-Westfälische Technische Hochschule Aachen University in Germany.
"It was amazing to me just seeing that that is possible," Kopper said.
But this is very much in keeping with the ethos of IceCube, an observatory built on international collaboration. IceCube is operated by a group of scientists, engineers and staff from 53 institutions in 12 countries, together known as the IceCube Collaboration. The project's headquarters is WIPAC, a research center of UW-Madison in the United States.
To confirm the detection and usher in a new chapter of neutrino astronomy, the IceCube Collaboration is working to detect more Glashow resonances. And they need IceCube-Gen2, a proposed expansion of the IceCube detector, to make it happen.
"We already know that the astrophysical spectrum does not end at 6 PeV," Lu said. "The key is to detect more Glashow resonance events and to identify the sources that accelerate those antineutrinos. IceCube-Gen2 will be key to making such measurements in a statistically significant way."
Glashow himself echoed that sentiment about validation. "To be absolutely sure, we should see another such event at the very same energy as the one that was seen," said Glashow, now an emeritus professor of physics at Boston University. "So far there's one, and someday there will be more."
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The IceCube Neutrino Observatory is funded primarily by the National Science Foundation and is operated by a team headquartered at the University of Wisconsin-Madison. 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. IceCube construction was also funded with significant contributions from the National Fund for Scientific Research -- the FNRS and FWO -- in Belgium; the Federal Ministry of Education and Research and the German Research Foundation in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the Department of Energy and the University of Wisconsin-Madison Research Fund in the U.S.
(Note for media: Please include a link to the original paper in online coverage: https:/
Tracking cosmic ghosts
Frontera supercomputer, a community resource for IceCube Neutrino Observatory research, enables the discovery of new high-energy particle
IMAGE: A VISUALIZATION OF THE GLASHOW EVENT RECORDED BY THE ICECUBE DETECTOR. EACH COLORED CIRCLE SHOWS AN ICECUBE SENSOR THAT WAS TRIGGERED BY THE EVENT; RED CIRCLES INDICATE SENSORS TRIGGERED EARLIER... view more
CREDIT: ICECUBE COLLABORATION
The idea was so far-fetched it seemed like science fiction: create an observatory out of a one cubic kilometer block of ice in Antarctica to track ghostly particles called neutrinos that pass through the Earth. But speaking to Benedickt Riedel, global computing manager at the IceCube Neutrino Observatory, it makes perfect sense.
"Constructing a comparable observatory anywhere else would be astronomically expensive," Riedel explained. "Antarctica ice is a great optical material and allows us to sense neutrinos as nowhere else."
Neutrinos are neutral subatomic particles with a mass close to zero that can pass through solid materials at near the speed of light, rarely reacting with normal matter. They were first detected in the 1950s in experiments that operated near nuclear reactors, which also generate these particles. They were further found to be created by cosmic rays interacting with our atmosphere. But astrophysicists believed they were likely widespread and caused by a variety of cosmic events, if only they could be detected.
Importantly, scientists believed they could be critical clues to other phenomenon. "20 percent of the potentially visible Universe is dark to us," Riedel explained. "That's mostly because of distances and the age of the Universe. High energy light is also hidden. It is absorbed or undergoes transformation that makes it hard to trace back to a source. IceCube reveals a slice of Universe we haven't yet observed."
An Important New Tool in the Multi-Messenger Astronomy Toolbox
Multi-messenger astronomy describes an approach that combines observations of light, gravitational waves, and particles to understand some of the most extreme events in the Universe. Neutrinos play an important part in this type of research.
Prior to 1987, with the explosion of Supernova 1987a, all extra-solar astronomical observations were photon-based. Today, additional detection systems add to our view of the cosmos, including all sky surveys and gravitational wave detectors. However, most observatories can only look at a small portion of the sky. IceCube, because of the nature of neutrinos, can observe these particles' flights from any direction, and therefore act as a full-sky sentinel.
The block of ice at the Amundsen-Scott South Pole Station in Antarctica -- up to a hundred thousand years-old and extremely clear -- is instrumented with sensors between 1,450 and 2,450 meters below the surface. As neutrinos pass through the ice, they may interact with a proton or neutron, producing photons which then travel through the ice, and can be detected by a sensor. The sensors transform these signals from neutrino interactions -- a handful an hour -- into digital data that is then analyzed to determine whether they represent a local source (Earth's atmosphere) or a distant one.
"Based on the analysis, researchers are also able to determine where in the sky the particle came from, its energy, and sometimes, what type of neutrino -- electron, muon or tau -- it was," said James Madson, executive director at the Wisconsin IceCube Particle Astrophysics Center.
In 2017, IceCube detected a neutrino with an energy of 290 teraelectronvolts (TeV) and sent out an alert. The detection triggered an extensive campaign involving more than twenty space- and ground-based telescopes. They identified a blazar 3.5 billion light years away, identifying a high energy cosmic ray source for the first time and launching a new era in multi-messenger detection, according to Riedl.
"We continuously search our dataset in near-real time for interesting neutrino events," he explained. "We found one and sent out an email alert to the community. They followed up with all these other electromagnetic observations, pinpointing a known gamma ray source. They also found, over the course of a month, an increased activity from the source."
CAPTION
The IceCube Neutrino Observatory is the first detector of its kind, designed to observe the cosmos from deep within the South Pole ice. An international group of scientists responsible for the scientific research makes up the IceCube Collaboration.
CREDIT
Yuya Makino, IceCube/NSF
IceCube Discovers Evidence of High-energy Electron Antineutrino
On March 10, 2021, IceCube announced the detection of a Glashow resonance event, a phenomenon predicted by Nobel laureate physicist Sheldon Glashow in 1960. The Glashow resonance describes the formation of a W? boson -- an elementary particle that mediates the weak force -- during the interaction of a high-energy electron antineutrino with an electron, peaking at an antineutrino energy of 6.3 petaelectronvolts (PeV). Its existence is a key prediction of the Standard Model of particle physics. The results further demonstrated the ability of IceCube to do fundamental physics. The result was published on March 10 in Nature.
While this energy scale is out of reach for current and future planned particle accelerators, natural astrophysical phenomena are expected to produce antineutrinos that reach beyond PeV energies. The news of the Glashow resonance discovery, "suggests the presence of electron antineutrinos in the astrophysical flux, while also providing further validation of the standard model of particle physics," the authors wrote. "Its unique signature indicates a method of distinguishing neutrinos from antineutrinos, thus providing a way to identify astronomical accelerators that produce neutrinos via hadronuclear or photohadronic interactions, with or without strong magnetic fields."
Neutrino detections require significant computing resources to model the detector behavior and differentiate extra-solar signals from background events created from cosmic ray interactions in the atmosphere. Riedel serves as the coordinator for a large community of researchers -- as many as 300 by his estimates -- who use the Frontera supercomputer at the Texas Advanced Computing Center (TACC), a National Science Foundation (NSF)-funded resource for the national community.
IceCube was awarded time on Frontera as part of the Large Scale Community Partnership track, which provides extended allocations of up to three years to support long-lived science experiments. IceCube - which has collected data for 14 years and was recently awarded a grant from NSF to expand operations over the next the next few years -- is a premier example of such an experiment.
"Part of the resources from Frontera contributed to that discovery," Riedl said. "There's years of Monte Carlo simulations that went into it to figuring out that we could do this."
IceCube uses computing resources from a number of sources, including the Open Science Grid, the Extreme Science and Engineering Discovery Environment (XSEDE), their own local supercomputing cluster, and recently the Amazon Web Services cloud. Frontera is the largest system utilized, however, and can handle a large part of the computational needs of the neutrino community, reserving local or cloud resources for urgent analyses, Riedel says.
"A lot of the computing on Frontera may not be directly associated with discoveries, but it helps down the road, to discern signals better and develop new algorithms," he said.
Modeling Ice and Following Up on Promising Signals
The projects that IceCube scientists use Frontera for vary, but they typically either involve calculations to better understand the optical nature of the ice generally (so the trajectory and other characteristics of neutrino detections can be accurately determined); or computations to analyze specific events that are deemed significant.
The first type of computation uses primarily ray tracing to calculate the path of the light in the ice from high-energy electrically charged particles produced when neutrinos interact. The rays can scatter or be adsorbed by defects in the ice, complicating analysis. Using graphics processing units (GPUs), Riedel has found, can speed up the simulations to studying light the light propagation in the ice by hundreds of times. The IceCube team is among the largest users of the Frontera GPU subsystem that includes NVIDIA RTX GPUs.
The second type of computation occurs when scientists receive an alert that says they have received an interesting signal. "We kick off a calculation to analyze the event that can scale to one million CPUs," Riedl said. "We don't have those, so Frontera can give us a portion of that computational power to run a reconstruction or extraction algorithm. We get those type of events about once a month."
"Large scale simulations of the IceCube facility and the data it creates allow us to rapidly and accurately determine the properties of these neutrinos, which in turn exposes the physics of the most energetic events in the universe," said Niall Gaffney, TACC Director of Data Intensive Computing. "This is key to validating the fundamental quantum-mechanical physics in environments that cannot be practically replicated on earth."
Today's astronomers can observe the universe in many different ways, and computing is now central to almost all of them. "We've moved from the traditional view of a guy with a telescope looking up at the sky, to large scale instruments, to now particle physics and particle observatories," Riedl said. "With this new paradigm, we need large amounts of computing for short periods of time to do big time sensitive computing, and big scientific computing centers like TACC help us do our science."
CAPTION
Flags outside of IceCube represent the international collaboration of the project.
CREDIT
Yuya Makino, IceCube/NSF
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