One of the most ambitious and extreme experiments on Earth opened at the South Pole in 2010. IceCube, a giant particle detector buried in the polar ice, captures elusive, high-energy species of neutrinos—fundamental particles that fly straight through almost everything they touch. The project, for which I am the principal investigator, aims to use neutrinos to study distant cosmic phenomena—particularly the mysterious, violent processes thought to produce the charged particles known as cosmic rays that continually bombard Earth.
We expect IceCube to catch these very high energy neutrinos only rarely. The particles have almost no mass and no electrical charge (which is why they seldom react with other particles), and they travel at nearly the speed of light. Once they arrive on Earth from near or far away, most do not stop to linger; they keep traveling, zipping straight through our planet and continuing on their way. Because of these difficulties, we were not surprised that the experiment's first few years of data, taken while the detector was still under construction, turned up nothing extraordinary. But in 2012 that changed.
One day, during a routine conference call for team members, our screen lit up with patterns we had never seen before. The signals reflected two neutrinos carrying more than 1,000 times the energy of the most energetic neutrino ever produced by an accelerator on Earth and almost a billion times the energy of the neutrinos regularly spat out by the sun. Clearly, they came from some spectacularly energetic process occurring far from our planet. Exhilaration spread through the room as we realized we were looking at something game changing. Capturing the whimsy of the moment, one of our graduate students nicknamed the two particles “Bert” and “Ernie,” after the Sesame Street characters (the names are not just fun; they are easier to keep straight than the long strings of numbers we usually assign to neutrino events).
It took us another year and a totally redesigned analysis of the same data to satisfy ourselves that these were indeed what they seemed: the first pixels of the first pictures of the distant neutrino universe. Since then, we have found 54 high-energy neutrinos in total—many of them given Muppet names, including one dubbed “Big Bird” with an energy twice that of Ernie or Bert.
We are now trying to identify where in the sky these high-energy neutrinos came from and how they originated. Their suspected sources are extreme cosmic events such as supernovae and other stellar explosions called gamma-ray bursts—two phenomena rumored to give rise to cosmic rays. If we can definitively trace the high-energy neutrinos to these likely sources of cosmic rays, we will open a new frontier in our understanding of the physics behind the extraordinarily dramatic events that are thought to produce them.
Cosmic rays, which constantly bombard Earth from space, are made of extremely high energy protons and other charged particles. More than a century after their discovery, the processes that birth them are still unknown. When they arrive at Earth, we cannot deduce where in the universe they came from because their electrical charge allows galactic and intergalactic magnetic fields to alter their course as they cross space. Luckily, however, theory suggests that cosmic rays also interact at their birthplaces with photons to produce neutrinos.
Neutrinos, unlike cosmic rays, do point back to where they started. Because they shun other matter, almost nothing can divert them from their path. Therefore, although cosmic rays themselves cannot lead us to where they began, the highly energetic neutrinos they presumably produce can do so for them.
Of course, astronomers have some ideas about how cosmic rays are born, but we need data to help us confirm or discard those possibilities. One probable source of cosmic rays is the death throes of massive stars. At the end of a large star's life, when its nuclear core can no longer support its mass, it will collapse into a dense object called a neutron star or into an even denser black hole, from which nothing escapes. In addition to creating a bright blast of light—a supernova—the collapse converts large amounts of gravitational energy into thrust for the acceleration of particles, presumably through shock waves. Supernova remnants were proposed as a likely source of cosmic rays as early as 1934 by astronomers Walter Baade and Fritz Zwicky; after 80 years, the hypothesis is still debated. About three supernova explosions in the Milky Way every century, converting a reasonable fraction of a star's mass into fuel for particle acceleration, could account for the steady flow of cosmic rays seen in the galaxy.
Extragalactic cosmic rays, which originate from beyond our home galaxy, are generally even higher in energy than the cosmic rays coming from nearby, and they require a more energetic source to create them. One contender is gamma-ray bursts. Even brighter than regular supernovae, gamma-ray bursts are somewhat mysterious but probably occur during a special class of star collapse that involves very high mass stars under extreme conditions.
Another theoretical source of extragalactic cosmic rays is active galactic nuclei—a class of galaxies suspected to have a supermassive black hole at their center that is absorbing large quantities of matter. As matter falls into such a black hole, some particles could be deflected outward and accelerated to high speeds to become cosmic rays.
To catch a neutrino
To detect neutrinos produced by the cosmic rays coming from such processes, IceCube has to be extraordinarily huge. The experiment uses a full cubic kilometer of 100,000-year-old Antarctic ice 1.5 kilometers below the surface of the South Pole for the job. Ice is a perfect natural neutrino detector because when a neutrino does occasionally interact with atoms in the ice, the material lights up by releasing a shower of charged particles that radiates blue light. This so-called Cherenkov radiation travels hundreds of meters through the pure, ultratransparent ice. IceCube is equipped with 5,160 optical sensors spaced throughout its volume to spot this light.
The sensors chart, in exquisite detail, the light pool produced by the nuclear debris created when a single neutrino hits. This pattern reveals the neutrino's type (or “flavor”), energy and arrival direction. The energies of Ernie and Bert and the others that we have seen so far are about a peta–electron volt (PeV), or 1015 eV; Ernie and Bert were 1.07 PeV and 1.24 PeV, respectively. For comparison, the particle beams at the Large Hadron Collider at CERN near Geneva, the world's most powerful particle accelerator, are in the tera–electron volt (TeV), or 1012 eV, range, about three orders of magnitude less. These energies made them the most energetic neutrinos ever found, by a wide margin. The light pool of roughly 100,000 photons created by Ernie and Bert extended over more than 500 meters, or about six city blocks.
Most important, the PeV energies of these two neutrinos tell us that they must be part of some cosmic signal—their energies are just too large to have been produced nearby. Local neutrinos are a dime a dozen. Every six minutes, IceCube detects a neutrino that is produced in the interactions of cosmic rays with hydrogen and oxygen nuclei in Earth's atmosphere. But these neutrinos, because they are made in our own backyard, are useless for telling us anything about the nature of cosmic rays or other astrophysical phenomena. We therefore have to screen out these distractions to detect cosmic neutrinos. From past experience, we know the light patterns produced by garden-variety neutrinos, so we ignore those.
Therefore, we can be quite sure the PeV-energy neutrinos that IceCube is seeing come from the distant cosmos. They very well could have reached us from the same sources as cosmic rays. But there are also other possible, more exotic explanations for these particles. One suggestion is that they may be signatures of dark matter—the invisible material that seems to make up more than 80 percent of all matter in the universe. That notion would be plausible if dark matter were made of very heavy particles with an average lifetime longer than the current age of the universe. In such a scenario, dark matter particles could occasionally decay to produce the PeV-energy neutrinos that we observe.
Before the discovery of PeV neutrinos, IceCube's search for cosmic neutrinos had almost exclusively focused on one of the three flavors of neutrinos: muon neutrinos (the others are electron and tau neutrinos). Scientists expect cosmic neutrinos to come in roughly equal numbers of the three categories when they reach Earth, but some are easier to look for because of the signal they produce in our detector. We had originally optimized IceCube to search for muon neutrinos that slammed into atoms primarily outside the detector's boundaries, producing kilometer-long light trails that would extend through the detector volume. This technique allows us to essentially expand our neutrino “collecting” area so that it is larger than the volume of the actual detector, but it also opens the door wider for contamination from particles other than cosmic neutrinos, so we must take extra measures to screen out this background.
We also ran another analysis, focused on searching for a particular class of extremely high energy neutrinos called Greisen-Zatsepin-Kuzmin (GZK) neutrinos, which arise from interactions between cosmic rays and photons from the cosmic microwave background, left over from the big bang. Such neutrinos would have energies in the range of exa–electron volts (EeV)—roughly 1018 eV.
This second search homed in on a more limited region of IceCube—the inner half of the detector, leaving less room for contamination to get in. The great advantage of confining the search in this way is that the detector can measure the full energy each neutrino deposits in the ice with about 10 to 15 percent accuracy—a big improvement on measurements we can make of neutrinos interacting outside the detector. We have yet to find GZK neutrinos, but this search has turned up plenty of cosmic neutrinos in all three flavors.
Since the discovery of Ernie and Bert, we have found more cosmic neutrinos through both this search method and our original plan of looking for muon neutrinos. Our first year's worth of data revealed 26 neutrinos with energies between 30 and 1,200 TeV, bringing our total to 28 when we include Ernie and Bert. This number is more than four standard deviations above what we would expect purely from the atmospheric background, meaning that the probability is greater than 99.9999 percent that these events truly come from deep space. When we later added an additional year of data, we brought the tally to 54 cosmic neutrinos and raised the significance of the signal to well over five standard deviations, the statistical threshold for a “discovery.”
Exactly where in the universe do all these neutrinos point back to? The events we have collected so far are not a large enough sample to provide a conclusive answer. They do not seem to be restricted to our galaxy—the sky map indicating their arrival directions shows only marginal evidence for an overlap with the plane of our galaxy; most come from directions far off the plane and are almost certainly of extragalactic origin. There does, however, appear to be a somewhat higher than average number of neutrinos coming from the center of the Milky Way. Bert, still one of the highest-energy neutrinos observed, is part of that cluster and points to within one degree of the galactic center. We cannot say for sure why this area is spewing out such numbers of neutrinos, but we know the galactic center is packed with supernova remnants, as well as a giant black hole, and thus holds many likely candidates for the neutrinos' source.
We hope to gain a better idea of where cosmic neutrinos originate as we steadily collect more muon-flavored neutrinos reaching us through Earth. Because these particles produce kilometer-length light trails, their directions can be reconstructed with better than 0.5-degree resolution, yielding a map of the sky that will be more revealing. This map will show where in the sky cosmic rays are coming from; if their directions happen to overlap with known objects in the sky, such as bright galaxies that host active galactic nuclei or gamma-ray bursts, we may be able to finally pinpoint some of the sources of cosmic rays.
IceCube is just beginning to scratch the surface of what it can discover. The experiment is built to operate for 20 years—maybe more. In the meantime, we are already looking toward its sequel. Our team is proposing to eventually build an expanded detector using roughly 10 cubic kilometers of ice—about 10 times the volume of IceCube. With that increased size, we may collect thousands more cosmic neutrinos to finally determine where they and their cosmic-ray cousins come from.