© Matt Strassler
IceCube is one of the world’s largest experiments, consisting of a cubic kilometer (that’s about a fifth of a cubic mile, or a billion tons) of ice, located well below the snowy surface at the south pole of our planet. It’s designed to detect neutrinos from outer space that happen to hit something within or just outside its immense volume. In this article I’ll tell you how a bit about how it works.
Elusive is the Neutrino
To understand the motivation for this experiment, it’s good to start with a little review of neutrinos. There are three types of neutrinos, and three types of anti-neutrinos, but for the moment, rather than talk about what distinguishes these types, let’s focus on the things that all of these neutrinos and anti-neutrinos have in common.
[To keep things short, I won’t write “neutrinos and antineutrinos” every time in this article. IceCube can’t tell the difference between neutrinos and anti-neutrinos, so unless otherwise specified, in this article I’ll just refer to them collectively as “neutrinos”. The same will be true for “muons”, which refers to “muons and anti-muons”.]
The main thing neutrinos have in common is that (unlike electrons and positrons and quarks and anti-quarks and photons and most other known particles) they are affected neither by the electromagnetic force nor by the strong nuclear force. [Here’s an article on the elementary particles and forces.] They are affected by the weak nuclear force. But this force is so weak that neutrinos can pass readily through large amounts of ordinary matter. This makes them quite unlike electrons, unlike photons, and unlike protons, neutrons and other hadrons. In fact, trillions of neutrinos are passing through your body every second, a fact that was unknown to us until recent decades. Since any attempt by humans to detect neutrinos (or, indeed, anything else) involves building some sort of device made from ordinary matter, the fact that neutrinos basically go right through ordinary matter without leaving any trace is rather inconvenient for anyone who wants to observe them.
Think how inconvenient it would be if photons went right through our photon detectors (you know, the things on our faces that we call “eyes”) without leaving a trace! Of course this is part of why we have eyes that detect photons, but we don’t have eyes that try to detect neutrinos — the latter wouldn’t be of much use.
Yet although the fraction of neutrinos that do hit something as they head through ordinary matter is extremely tiny, it’s not quite zero, so it is in fact possible to detect them. Nevertheless, there’s no choice but to
- obtain a very big piece of material to serve as a neutrino stopper,
- put scientific instruments all around or inside it, and then,
The scientist has to play the role of the patient spider with a very big web, or of a laconic fisherman with a vast net, awaiting that rare neutrino that isn’t looking where it’s going and blunders into the trap.
And this is where IceCube comes in.
IceCube: A Billion Tons of Ice
IceCube is, truly, the stuff of science fiction turned into fact. It is a giant cube of ice — pure, clear ice, free of bubbles and almost perfectly transparent. On our planet, such a giant quantity of perfect ice can only be found in the high plains near the South Pole, deep beneath the surface. And into that ice, scientists have repeatedly drilled two and a half kilometers (more than a mile and half) deep, using nothing more clever than hot water. As illustrated in Figure 1, into each drill hole (half a meter [a foot and a half] wide), scientists have placed a long string, the bottom kilometer of which is studded with sixty evenly-spaced instruments, called “phototubes”, that are able to observe very small amounts of light. Way down there, over a thousand meters into the ice (many thousands of feet), it’s incredibly dark. No light from the Earth’s surface makes it to that depth. So anything that a phototube can observe in the pitch darkness must be from something special.
What possible source of light could there be in a lifeless brick of pure ice? The only source is particle physics. When an elementary particle carrying a large amount of energy zips into IceCube and strikes the nucleus of an atom in the ice, it creates a shower of particles, traveling near the speed of light c — slower than the speed of light in empty space, but faster than the speed of light in ice. Electrically-charged particles that travel beyond the speed of light in the material through which they pass emit a dim cone of light, called Cerenkov radiation. [It’s much the same way that supersonic aircraft emit a sonic boom.] Dim as it is, this tiny flicker of light can traverse the ice for many meters, to be detected by the phototubes. And the pattern of the light observed by the phototubes can tell scientists
- roughly how much energy was deposited in the ice, giving an even rougher estimate of how much energy the particle was carrying
- roughly where the particle was traveling from, and
- certain limited information about the type of particle that created the collision, and the type of collision that occurred.
This is illustrated in Figure 2, where two neutrinos are coming from the sky, one of them making a shower of particles that doesn’t include a high-energy muon going out, and one of which makes a shower that does include a high-energy muon going out. [The latter can happen when the incoming neutrino is a muon neutrino, which can be converted by the weak nuclear force into a muon during a collision. For more details on the collisions between neutrinos and atomic nuclei, see this article.] The same is shown in Figure 3, for neutrinos that entered the other side of the earth and, from IceCube’s point of view, are coming up from the ground. The reason to have both figures will become clear later.
The only types of particles that can pass through over a kilometer of ice, or even a large fraction of the entire earth, to make it to IceCube are neutrinos and muons (and anti-muons — just as I’m doing for anti-neutrinos, I won’t bother to say “anti-muons” every time). So the only thing IceCube has to do, in order to detect neutrinos, is to make sure that it can distinguish their effects from those of muons. How that’s done is partly illustrated in Figure 6 below, where you see that the incoming muon deposits light as it enters IceCube, whereas an incoming neutrino doesn’t. We’ll come back to this point later.
That’s really all there is to IceCube. Well, almost. I should also mention that there are a few phototubes (i.e. light detectors) placed at the surface of the ice too, in a cluster called “IceTop”; the purpose of these will become clear when we get to Figure 7. Still, by comparison with the detectors at the Large Hadron Collider, which it far exceeds in size, IceCube is a very simple detector.
The technology IceCube uses was tried out on a chunk of ice more than ten times smaller, in an experiment called AMANDA. And a similar idea (using Cerenkov radiation created by collisions of neutrinos with matter) has been used in big tanks of water instead of ice. But you can imagine it’s not easy to get yourself a tank of water as big as IceCube. In fact the only way to get so much water is to use an ocean — which is what an experiment called ANTARES is doing, putting strings of instruments deep underwater. Ice has some disadvantages — once the phototubes are put into the warm-water holes and the holes freeze, there’s no way you can get them out again to do any repairs if something breaks. On the other hand, in the ocean there are currents that will move your detectors around, and a lot of living creatures make light that will distract you from what you’re trying to observe. In any case, it’s good that there are multiple experiments using different methods, so that we can compare their results and figure out which aspects can be trusted.
What IceCube Does (or May Soon) Observe
What natural processes generate neutrinos and anti-neutrinos that IceCube can measure? IceCube can detect and measure the energy of neutrinos whose energies exceed about 100 GeV — about the mass-energy [i.e. E=mc² energy] of a Higgs particle. Now what’s that good for?
Well, first, here are some neutrinos IceCube is not good for: the more common, lower-energy neutrinos that come from decays of unstable atomic nuclei in the rock (an example of “radioactivity”), neutrinos from the sun’s internal furnace, and neutrinos from the heart of a supernova explosion. These neutrinos have energies of a few thousandths of a GeV to a few hundredths of a GeV, far below what IceCube can detect efficiently. None of these are IceCube’s targets.
By far the most common neutrinos that IceCube observes are “atmospheric neutrinos”. These are actually produced in the atmosphere of the Earth by cosmic rays. Cosmic rays are very high energy particles, typically protons, that hit an atomic nucleus in the high atmosphere, thereby creating a shower of hadrons (the general term for particles made from quarks, antiquarks and gluons), along with photons, electrons and positrons [ie. anti-electrons]. Some of the hadrons then in turn can produce neutrinos when they decay. These neutrinos have a huge range of energies, with the number decreasing rapidly as the energy increases. They may come from all around the Earth, as shown in Figure 5; the cosmic rays come in from all directions, and hit the Earth’s atmosphere at all locations above the Earth’s surface, so they can create neutrinos that can reach IceCube from every point above the surface of the earth. An interesting fact of geometry is that IceCube (and any similar detector) observes equal numbers of these neutrinos coming from all directions — although the ones that come from underground (i.e. from the north in IceCube’s case) traverse thousands of kilometers (miles) of rock, while those that come from overhead travel through scarcely a kilometer (mile) or two of ice before reaching the detector.
There’s a challenge for measuring the neutrinos that come from overhead. Neutrinos and muons are both very common in a cosmic ray shower, but a muon is much more likely to produce a signal in IceCube than a neutrino is, so most of what IceCube sees coming from overhead is effects of muons, not neutrinos. Most of the time this signal is clearly that of a muon and not a neutrino (Figure 6), but every now and then something odd will happen, and the light that distinguishes an incoming muon from a neutrino won’t get picked up. In this case, IceCube will measure a “fake neutrino” that was really a muon. Even though this is rare, there are so many muons that these fake neutrinos do have to be accounted for, especially at low energy.
Fortunately, most of the cosmic-ray muons don’t make it down a kilometer and a half into the ice. And even more fortunately, muons that are coming from cosmic-rays that are below the horizon at the South Pole have to travel through many hundreds or thousands of kilometers of Earth to get to IceCube — and they won’t make it. So therefore, while muons can create fake downward-going neutrinos (downward meaning, as usual, “toward the Earth’s center”), they can’t create fake upward-going neutrinos. So upward-going neutrino signals are pure — essentially always from real neutrinos.
Now, what about other neutrinos? The ones that IceCube is most likely to detect, and that are its main target, are called “astrophysical neutrinos”. What are these? Well, one thing we are confident of, but don’t understand in detail, is that out there in space somewhere, something truly dramatic is responsible for making the very highest-energy cosmic rays that we observe — protons and atomic nuclei with energies approaching a million million GeV, and probably beyond. (Remember the Large Hadron Collider only accelerates protons to energy of a few thousand GeV.) Now, it is almost impossible to imagine how this acceleration process could occur without there being collisions, within this distant natural particle accelerator, between these protons and other matter that they may encounter. And those collisions would create neutrinos, with energies somewhat lower than the ultra-high-energy cosmic rays, but still higher than any neutrinos previously observed. We don’t know how many of those neutrinos might be out there, but we have reason to believe that there might be enough for IceCube to detect… and maybe it already has done so?
The main goal of IceCube is to discover astrophysical neutrinos and help figure out, if possible, what is making them. This requires not only observing the neutrinos but also looking, with ordinary and exotic telescopes, for photons (visible light, or radio waves, or gamma rays — anything!) that are coming from the same place on the sky, perhaps at nearly or exactly the same time. For instance (see here and here for an example), if a big explosion somewhere out there in space generates big magnetic and electric fields that can accelerate protons and electrons to extreme energies, various interactions between those protons and electrons and other gas that is flying about may generate both high-energy photons and very high-energy astrophysical neutrinos. With luck, the photons could be detected by one or another special telescope at roughly the same time, and from the same direction, as IceCube detects a neutrino. [Yes, probably just one astrophysical neutrino would be observed at a time.] This hasn’t happened yet, somewhat to everyone’s disappointment, but it is still early days at IceCube.
Anyway, the point is that if IceCube does in fact see astrophysical neutrinos, it will start to serve just like any other telescope, allowing us to observe, with admittedly blurry neutrino-vision, exactly where the sky is “bright” in high-energy neutrinos. Historically, every time we add a new type of telescope, we discover new types of objects out in space, and learn more about the objects we’ve discovered previously. So that is the hope for IceCube… that it will soon be (or has already become) a neutrino telescope looking not at cosmic rays hitting the atmosphere (via atmospheric neutrinos) but at extremely energetic objects far away across the vast expanses of space (via astrophysical neutrinos.)
Looking Up as Well as Down for Astronomy
Atmospheric neutrinos come from overhead, along with large numbers of muons, a few of which make fake neutrinos. The number of such neutrinos and muons becomes small at high energy, so that sufficiently high-energy astrophysical neutrinos (with energies approaching a million GeV!) can be observed rather easily. But astrophysical neutrinos with energies well below a million GeV are lost in the sea of atmospheric neutrinos. Fortunately, there’s a trick for reducing the number of atmospheric neutrinos and atmospheric muons coming from overhead, and this can allow IceCube to detect astrophysical neutrinos coming from overhead down to energies of perhaps 10,000 GeV. This technique was used (along with other refinements) in IceCube’s recently announced and exciting data. The two highest-energy neutrinos in that data had so much energy that they were unlikely to be atmospheric in origin. But the remaining 26 (of which only 10 were expected) were identified only after using this trick.
The trick is to set aside those downward-going neutrinos that are almost certainly atmospheric in origin, retaining only the astrophysical neutrinos, a few of the atmospheric ones, and a few fake neutrinos from atmospheric muons. While this can’t be done with complete reliability, it can be attempted as shown in Figure 7. The muons from a cosmic ray shower (Figure 4) arrive at IceTop (Figure 1) at the same time as the neutrino arrives at IceCube, to within a few hundred thousandths of a second. If this happens, the neutrino was almost certainly atmospheric. If not, then it probably (but not certainly) wasn’t. Throwing away the neutrinos that were clearly atmospheric reduces the number of neutrinos by a lot, but the IceCube folks claim they still see more neutrinos than they would have expected. Stay tuned over the coming years as the IceCube folks try to understand what this means. Have they in fact detected astrophysical neutrinos?