L&S researchers are using the IceCube Neutrino Observatory to shed light on some of our universe's greatest mysteries … and there's more to come
Deep in the Antarctic ice, all is seemingly calm.
Yet particles — tiny, subatomic particles — are whizzing by at an astonishing rate. And when the intergalactic guest of honor finally arrives, chaos ensues.
A violent collision sends millions more particles flying.
It's a moment of scientific discovery — happening in a flash — and University of Wisconsin-Madison researchers are watching. And each of these moments, each time the scientists spot a mysterious neutrino from the cosmos, gives them more data, more clues in their quest to better understand our universe.
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The term "neutrino" might sound straight out of science fiction — and there are numerous references across the Star Trek universe, not to mention in Ghostbusters and Doctor Who— but the nearly massless subatomic particles are allowing physicists in UW-Madison's College of Letters & Science to look at the sky like never before … even if more questions than answers are staring back at them.
UW-Madison has become one of the world's top spots to study particle astrophysics, a field that, as its name suggests, is at the intersection of particle physics and astronomy.
The driving force is the IceCube Neutrino Observatory, a first-of-its-kind particle detector that's buried in a cubic kilometer of Antarctic ice and run by an international collaboration led by UW-Madison. IceCube has already discovered the first evidence for cosmic, high-energy neutrinos coming from outside our solar system and may help UW-Madison scientists pursue even greater astrophysical mysteries such as dark matter.
Francis Halzen, a native of Tienen, Belgium, came to UW-Madison in 1971. He is the principal investigator of IceCube. (Photo by Bernardo Perez, El Pais)
Francis Halzen, the Hilldale and Gregory Breit Distinguished Professor of Physics, is the principal investigator of IceCube and one of five UW-Madison physics professors involved in the collaboration. The Belgian physicist has dedicated the majority of his career to hunting neutrinos, a pursuit that's sent members of his research collaboration to — literally — the ends of the Earth.
In November 2013, the international IceCube Collaboration published the landmark cosmic neutrino results, a feat that won a Breakthrough of the Year award from Physics World magazine and landed Halzen a 2014 American Ingenuity Award from Smithsonian magazine (presented by famed astrophysicist Stephen Hawking). The discovery represented one of the most significant steps to date in the quest to answer some of the so-called ghost particle's riddles, prompting Halzen to declare "the dawn of a new age of astronomy."
And he and the IceCube team have even bigger plans for the coming years: a larger detector capable of finding more — and higher-energy — neutrinos, gathering more evidence to decode cosmic neutrinos and their origins. This is basic research in its purest form, seeking to expand our fundamental knowledge of the universe and its cosmic processes.
It's also uncharted scientific territory: Even what sort of information neutrinos may hold remains a mystery.
"We'll find out, right?" Halzen says with a grin.
What, exactly, is a neutrino?
Neutrinos are subatomic particles, like the more familiar protons, neutrons and electrons we all learned about in high school chemistry class. However, protons and neutrons are composite particles, meaning they're made up of other, smaller particles. Neutrinos (and electrons), on the other hand, are elementary particles — the smallest unit of particle, as far as we know.
Under the Standard Model of physics — a framework explaining the known particles and forces, formulated in the 1970s — neutrinos are classed as leptons, of which there are six "flavors:" electrons, electron neutrinos, muons, muon neutrinos, tau and tau neutrinos.
Unlike electrons, though, neutrinos — regardless of the variety — hold no electric charge, which is the reason they're so darn difficult to spot. Billions pass through the Earth each second, unbeknownst to us, since they so seldom interact with matter (hence, the "ghost particle" moniker).
Wolfgang Pauli, an Austrian-Swiss theoretical physicist, first suggested the existence of the neutrino in 1930. Four years later, Italian physicist Enrico Fermi gave Pauli's particle its name, which means "little neutral one" in Italian.
But it wasn't until 1956 that American physicists Clyde Cowan and Frederick Reines detected neutrinos at a nuclear reactor in South Carolina using tanks of water and a compound called cadmium chloride. Since then, scientists have studied neutrinos with particle accelerators and aluminum plates, Olympic-sized swimming pools of chlorine, photomultiplier tubes submerged in water, tanks filled with mineral oil, and other gargantuan setups around the world.
Halzen began contemplating neutrinos as a young professor in the 1970s by working on a theory for a method to detect them in ice, based on an idea put forward by Soviet scientists a decade earlier.
A few years later, he began to build a research collaboration around the idea and, suddenly, a theoretical physicist who had never run a major experiment was on his way to leading the world's largest particle detector.
One of IceCube's 5,160 digital optical modules is deployed in the Antarctic ice. (Photo courtesy IceCube/NSF)
How to find them
The key difference between IceCube and previous neutrino detectors is its ability to find high-energy neutrinos — the kind that can only come from cosmic events.
Neutrinos have a variety of sources, including the sun (which emits neutrinos as a result of nuclear fusion, accounting for most of the neutrinos that pass through the earth), some radioactive isotopes and human creations such as nuclear reactors and particle accelerators.
Then there are the violent, powerful events that scientists believe yield the sort of neutrinos Halzen and his colleagues are chasing: black holes at the centers of galaxies, gamma ray bursts, supernovas, star formations and pulsars among them.
Suddenly, the Star Trek references don't seem so out of place.
"Neutrinos are really sort of the last stand, really the only known particle that we can use to reach out and look at the sky at extreme energies," says Kael Hanson, a professor of physics and the director of the Wisconsin IceCube Particle Astrophysics Center (WIPAC), the organizational container for IceCube, the Askaryan Radio Array (another neutrino detector, currently in its infancy) and several gamma ray experiments.
Other detectors previously picked up neutrinos traced to a supernova in a nearby galaxy in 1987, but not at the same sorts of energy levels observed by IceCube.
So how does IceCube snag these rare high-energy neutrinos?
For starters, it's all about location. The Antarctic ice is extremely clear and abundant — IceCube covers 1 billion tons of ice — and, at the detector's depth of 1 to 1½ miles, it's partially sheltered from the constant bombardment of cosmic rays that pummel Earth's atmosphere, showering particles everywhere.
It's also nice and dark. That's important, because when neutrinos travel through the ice, they sometimes — extremely infrequently — slam into a proton or neutron from a water molecule, producing a tiny blue light (called Cherenkov light after the Soviet scientist, Pavel Cherenkov, who discovered this electromagnetic radiation).
IceCube captures that light using its 5,160 digital optical modules (DOMs) — sensors the size of basketballs that are encased in glass and suspended on 86 cables, which provide power and allow the DOMs to send information about each collision to the surface.
As you'd expect, embedding the DOMs was no easy task. The UW Physical Sciences Lab in Stoughton, Wis., designed a special drill for the job, one that used hot water to bore deep into the ice.
That was just one part of the seven-year construction process that culminated in December 2010, one that was slowed by the reality that workers only had a few months each year during the Southern Hemisphere's summer. The National Science Foundation covered the bulk of the $279 million total cost, with the rest coming from partners around the world.
'Golden events'
But even with cutting-edge technology at work, interesting neutrinos — at least ones that will pique Halzen and his colleagues' interest —are few and far between.
Kael Hanson, the director of the Wisconsin IceCube Particle Astrophysics Center, previously worked on instrumentation, data acquisition and detector operations at IceCube. (Photo by Sarah Morton, College of Letters & Science)
IceCube picks up around 3,000 particle events a second, the vast majority simply the result of those pesky cosmic rays. A neutrino shows up about every six minutes, but almost all are low-energy types that don’t merit further investigation.
A couple of times each month, though, a neutrino carrying thousands of times the energy makes an appearance. These are the ones IceCube researchers care about, and the shape of the Cherenkov light tells them both the energy of the neutrino and the direction from where it came.
"These are golden events," Halzen says.
The first two came in May 2010, and their appearance caught the IceCube team by surprise. Halzen's group was searching for neutrinos possessing a million times higher energy than atmospheric neutrinos when "we saw these two things" — not the ultra-high energy neutrinos they were looking for, but certainly not the standard neutrinos they had seen every six minutes.
Enter Bert and Ernie, so named by IceCube's postdoctoral researchers and graduate students. Over the next two years, IceCube detected 26 more neutrinos of similar energy levels, and it continues to spot about 25 "golden events" each year.
This is how IceCube "sees" neutrinos. "Ernie" was the highest energy neutrino ever observed when it was detected Jan. 3, 2012. (Image courtesy IceCube Collaboration)
Into the darkness
IceCube's groundbreaking data is allowing UW-Madison researchers to delve into fundamental scientific questions, from investigating cosmic processes to better understanding neutrinos themselves.
That the particles can actually change flavors (electron, muon or tau) throughout their intergalactic journeys, a phenomenon called oscillation, only adds to their mystery. So, too, does the fact that neutrinos having mass — backed up by evidence of these oscillations in Japan in 1998 — doesn't actually jive with the Standard Model of physics. Neutrinos may even hold clues as to why our universe is built of matter rather than antimatter, part of the dizzyingly confusing concept of leptogenesis.
— Francis Halzen
And then there is the potential connection between neutrinos and dark matter, a mysterious, invisible substance that scientists believe makes up nearly 27 percent of our universe. Normal matter accounts for just less than 5 percent, while equally necromantic-sounding dark energy fills the remainder, roughly 68 percent.
Dark matter remains an enigma, since no one has detected it. But IceCube is on the lookout, searching for high-energy neutrinos coming from the sun — the result of dark matter particles falling into the sun and annihilating each other.
"I think if you asked people: What are the odds? Who's going to discover dark matter? I think we would be high in the poll," Halzen says.
Neutrino radio
There's more than one way to find high-energy neutrinos — just ask Professor Albrecht Karle, a German physicist who came to UW-Madison in 1997 as an assistant scientist to work on the Antarctic Muon and Neutrino Detection Array (AMANDA), the pilot project that preceded (and was eventually integrated into) IceCube.
Professor of Physics Albrecht Karle is the principal investigator of the Askaryan Radio Array. (Photo by Sarah Morton, College of Letters & Science)
He's worked on IceCube since the project's inception, but has now turned his attention to another experiment occurring a few miles away at the South Pole: the Askaryan Radio Array (ARA), for which he is the principal investigator.
ARA's purpose is to find even higher-energy neutrinos — ones that are about 100 times more energetic than Bert and Ernie — by measuring radio waves that are emitted when neutrinos collide with a water molecule’s nucleus deep in the ice. That interaction produces what's called a cascade of electrons — millions of them — that, in turn, creates a radio pulse that can be detected by antennas closer to the surface. The process is named after another Soviet scientist: Gurgen Askaryan, who theorized it in 1962.
"IceCube uses ice because it's transparent to optical light. ARA likes ice because cold ice is extremely transparent to radio waves,” says Karle.
ARA is still very much in the early stages, while it awaits additional funding. Three stations, each equipped with 16 antennas, are currently in the ice, but the project's aim is to have 37 stations — initially, at least — arranged in a hexagonal pattern, allowing it to cover an area 100 times larger than IceCube.
Once completed, ARA's first goal would be to answer a straightforward question: Just how many neutrino events are occurring at these higher energy levels? The answer could shed light on not only neutrinos and their sources, but the makeup of the highest energy cosmic rays in our universe.
'The quest continues'
While ARA adds another tool to WIPAC's neutrino arsenal, the IceCube Collaboration has exciting plans of its own. It's proposing IceCube-Gen2, a detector that's 10 times larger than the current setup, built in the same location, on top of the existing detector.
The reason? Scientists believe neutrinos may carry information about the events that spawned them, since they travel light years unscathed. But to begin to unravel the clues, researchers simply need more neutrinos. And what better way to find more than by building a larger detector?
"We're right at the precipice of understanding what's going on," says Hanson, who has worked on instrumentation, data acquisition and operations during his time with IceCube. "IceCube was a discovery instrument. We've discovered our signal, and now we need to go to the next phase to understand what the signal is."
An artistic rendering of the Antarctic surface, showing the position of the 86 strings of sensors in IceCube and the possible grid of the next-generation detector. IceCube-Gen2 would be 10 times larger than the current setup. (Image courtesy IceCube Collaboration)
Among the mysteries: Where do cosmic neutrinos originate from? IceCube's have come from all across the sky. Halzen compares the situation to creating a digital image of the cosmos — in essence, instead of seeing the sky in light beams, IceCube sees it in neutrino beams.
"We don't have enough pixels yet to make an image," he says. "Each neutrino is a pixel."
However, it's one thing to see where a neutrino comes from; it's a whole other challenge to determine what produced it. IceCube has sped up its data analysis to the point where researchers can spot an event in real time and alert astronomers, who can use high-powered telescopes to investigate the source.
It's a real-time version of an approach used by Assistant Professor of Physics Yang Bai and a team of UW-Madison physicists and astronomers last year. By analyzing historical data from IceCube and three NASA X-ray telescopes, Bai and his colleagues found evidence that Sagittarius A*, the enormous black hole at the center of the Milky Way, may have produced some of IceCube's high-energy neutrinos.
But, as Halzen notes, "it would be a great surprise if any of these neutrinos came from a steady source."
"They are usually things that explode and go 'boom,'" Halzen says. "The best bet now is that they come from sources not in our galaxy, throughout the universe."
Certainty is hard to find in the world of neutrinos. But that's precisely why IceCube researchers keep chasing the ghost particle — the promise that it will reveal new lessons about the universe and continue to spur astrophysics, and science as a whole, forward.
"There are practical aspects to astronomy, such as time keeping," says Hanson. "But that's not where we, as humans, started. It was curiosity. We wanted to know, what are these things that are going on up there? And we started realizing patterns. And this is just the quest. The quest continues."
"IceCube was a shot in the dark," adds Halzen. "If we had never seen anything, nobody would be surprised, including us. It was a total shot in the dark. But that's how science is, right? If it's not a shot in the dark, it's not science."
Sagittarius A*, the black hole at our galaxy's core, may have produced some of IceCube's high-energy neutrinos. The black hole is located in the bright white section of this composite image, which combines optical, infrared and X-ray NASA observations of the Milky Way's central region. (Image courtesy NASA - X-ray: NASA/CXC/UMass/D. Wang et al.; Optical: NASA/ESA/STScI/D.Wang et al.; IR: NASA/JPL-Caltech/SSC/S.Stolovy)