Antarctica's Ghost Hunters: Inside the World's Biggest Neutrino Detector

Gizmodo

3 days ago

Neutrinos are strange little things. This tiny, enigmatic particle with no charge exists in virtually every corner of the universe, but without powerfully sensitive, sophisticated instruments, physicists would have no way of knowing they exist. In fact, trillions are passing through you every second.
Physicists devise all sorts of ways to coax neutrinos into the detection range. But IceCube—which celebrates its 20th anniversary this year—stands out in particular for its unique setup: 5,160 digital sensors latched onto a gigantic Antarctic glacier. Recently, the IceCube Collaboration set the most stringent upper limits on a key statistic for ultra-high-energy neutrinos, often found inside cosmic rays. It's also slated to receive some major technological updates later this year, which will make the detector—already one of the largest neutrino experiments on Earth—even larger and stronger than ever.
Gizmodo reached out to Carlos Argüelles-Delgado, an astrophysicist at Harvard University and a neutrino expert who's been with IceCube since 2011, when the experiment began its physical operations in Antarctica. We spoke to Argüelles-Delgado about why IceCube is, well, in Antarctica, some highlights from the experiment's 20-year run, and what we can expect from the forthcoming IceCube-Gen2. The following conversation has been lightly edited for grammar and clarity.
Gayoung Lee, Gizmodo: Let's begin with the elephant in the room. Why did physicists decide Antarctica was a good place to find neutrinos?
Argüelles-Delgado: Yeah. You have a combination of two very difficult problems. You're looking for something that's very rare that produces very small signals, relatively speaking. You want an environment that is very controlled and can produce a large signal at a small background.
The IceCube project—kind of a crazy project if you think about it—the idea is we're going to take a glacier about 2.5 kilometers [1.5 miles] tall, which is one of the most transparent mediums that exists on the planet. We're going to deploy these very sensitive light sensors [called digital optical modules] that can detect single light particles known as photons. And so, you have this array of light detectors covering 1 cubic kilometer [0.24 cubic miles] of essentially pitch-dark space. When a neutrino comes from outer space, it can eventually interact with something in the ice and make light, and that's what we see.
Gizmodo: It's really difficult to understand what neutrinos actually are. They sound like something churned by particle physicists, but at other times they're discussed in the context of experiments like IceCube, which searches for neutrinos from space. What exactly are neutrinos? Why does it feel like they appear in every niche of physics?
Argüelles-Delgado: That's a good question. One reason that neutrinos appear in very different contexts—from particle physics to cosmology or astrophysics—is because neutrinos are fundamental particles. They are particles that cannot be split into smaller pieces, like the electrons. Like we use electrons in laboratories, we also use electrons in detecting physical phenomena.
Neutrinos are special because we have open questions about their behaviors and properties [and] about the universe in the highest energy regime, where we observe cosmic rays. So, observing neutrinos in a new setting on a new energy scale is always very exciting. When you try to understand something with a mystery, you look at it from every angle. When there's a new angle, you then ask, 'Is that what we expected to see? Is that not what we expected to see?'
Gizmodo: In the spirit of attempting new angles to solve mysteries, what sets IceCube apart from other neutrino detectors?
Argüelles-Delgado: It's huge! IceCube is a million times larger than the next neutrino experiment that we have built in the laboratory. It's just huge. Since the interaction rate depends on how many things you're surveying, the larger the volume, the more likely you are to see something. For ultra-high-energy neutrinos [which originate in space], you're always thinking about natural environments—mountains, glaciers, lakes—landscapes converted into experiments.
Gizmodo: Antarctica isn't exactly somewhere you can just fly to with a plane ticket. What challenges come from the unique conditions at this remote location?
Argüelles-Delgado: You're right. The logistics are very complicated. You have to ship all of these components, and you have to be sure that when you put something in the ice, it will work. It's like when you put something on a satellite on a spaceship. Once it's there, you cannot fix it. It is just there. So the quality requirements are very high, and there are multiple challenges. One of them is drilling the actual holes, through two steps. A mechanical drill makes the first guiding hole. Then we use a custom-made, high-pressure hot water drill that then pumps water to [carve the glacier]. The other part is the cable. The cables in IceCubes are quite special, [holding] the instruments that digitalize the modules, which allows you to have better quality of signal processing.
Gizmodo: What are the upcoming upgrades to IceCube, and why are they needed?
Argüelles-Delgado: The upgrade has two functions. One of them is that we need to better understand the glacier where IceCube is embedded. Obviously, we didn't make that glacier. We just put things on the glacier. And the better we understand the glacier and its optical properties—how light travels in that glacier—the better we can actually do neutrino physics. So we're going to install a bunch of cameras and light sources to try to sort of survey the glacier better.
We're also installing a bunch of new sensors [for] a larger version of IceCube, called IceCube-Gen2. When you do science, you want to test new things but also measure things. We are not going to be able to extend the detector volume, but instead we're going to put [sensors] in the innermost part of the detector to allow us to better measure lower-energy neutrinos in IceCube.
Low-energy neutrinos are important because at low energies, the neutrinos experience something known as flavor oscillation, which means that the neutrinos, as they travel from one side of the planet to the other, change in type. That is actually a quantum mechanical phenomenon of microscopic scales. IceCube shows one of the best measurements of that physics.
Gizmodo: You've been a part of IceCube's journey since the beginning. In your view, what are some of the main highlights the experiment has accomplished?
Argüelles-Delgado: First, we discovered that there are ultra-high-energy neutrinos in the universe. These are difficult to detect but not that rare in terms of the universe's energy density, or how much energy per unit volume exists in the universe between protons, neutrinos, and light—they're actually very similar—and IceCube established [this relationship]. A few years ago we saw the first photo of our galaxy in neutrinos.
Something very close to my heart is flavor conversion in quantum mechanics. We think neutrinos are produced primarily as electron- and muon-type neutrinos. Now as they travel through space, because of these quantum mechanical effects, they can transform into tau neutrinos, which are not initially there at production. In IceCube, we have found significant evidence of various tau neutrinos at high energy levels. What's amazing about this is that those neutrinos can only be produced and can get to us if quantum mechanics is operating at these extremely long distances.
Gizmodo: Given these highlights, what are some things that you are most looking forward to next?
Argüelles-Delgado: There are two things that I find very exciting in neutrino astrophysics. One is the neutrinos' quantum behavior, and we do not understand how they acquire their masses. Most particles, when they have mass, have two states that interact with the Higgs boson to produce their masses. Neutrinos, for some reason, we only see one of these states. What I'm excited about is looking for new flavor transformations of very high-energy neutrinos. In some of these scenarios, we could actually have some idea about neutrino mass mechanisms.
The second thing is, we have seen neutrinos that are 1,000 times more energetic than the product of the LHC [particle beam]. So are there more at the higher end of neutrinos? Is this where the story ends? What's interesting is that an experiment called KM3NeT in the Mediterranean has reported observations of a neutrino that's [100,000] times more energetic than the LHC beams. I think that is weird. You know, when you see weird things happening, it often means you don't understand something. And so we need to understand that puzzle.
Gizmodo: On a scale of 1 to 10, how likely is it that we'll solve these mysteries?
Argüelles-Delgado: If we discover the nature of neutrino masses is due to this quantum oscillation phenomenon of the high energies, this will be like a Nobel Prize discovery. Because it's such a big thing, I'll give you at best 1%.
Gizmodo: I'd say that's actually pretty good.
Argüelles-Delgado: I'd say that's pretty good, yeah. Let's say 1%. I think we'll solve the puzzle of the ultra-high-energy regime; that's a matter of time. That's going to take us easily another 15 years, but it's going to be, again, completely new land. We will see what awaits us. When IceCube started seeing the first neutrinos, we were so confused because we were not expecting to see them [like] this, right? And if all the confusion keeps happening, we'll find more interesting results