The IceCube detector, located at the South Pole, monitors a cubic kilometer of ice for the flashes of light produced as energetic particles traverse the ice. Each second, about 3,000 muons, produced by cosmic rays slamming into the atmosphere, interact with matter in the detector. In contrast, neutrinos are only detected once every six minutes.
Francis Halzen, the principal investigator for IceCube, described the search for these particles in the detector at the recent meeting of the American Association for the Advancement of Science. "It's like doing astronomy, but the sky is cloudy," he said. "It's cloudy all the time." Even the majority of the neutrinos that arrive at the detector aren't especially interesting; they're also produced as part of cosmic ray particle showers. Instead, the computers behind the detectors have to sort through 100 billion muons each year, along with 100,000 atmospheric neutrinos, just to find about 10 interesting events.
But the interesting events are incredibly energetic. "When it arrives, it hits your detector like a hammer," Halzen told the audience. "You don't have to look for it; it just announces itself." (The same goes for some of the energetic muons, two of which have deposited over 560 Tera electron Volts in the detector—compare that to the LHC's upcoming 14TeV collisions.)
In part due to the small numbers it detects, IceCube has mostly told us that incredibly energetic neutrinos exist. And we can work back from that knowledge to appreciate that there are incredibly energetic processes that must produce these neutrinos—"hadronic accelerators create a lot of the energy in our Universe" is how Halzen put it. But to start figuring out where in the sky these neutrinos originate, and thus what might be creating them, we need to get better at capturing more of them.
But Halzen has a plan. The ice beneath the South Pole turned out to be much better at transmitting the light from neutrino interactions than we'd expected. They now think they can take the same number of detectors (there are 5,160 of them) and spread them over 10 cubic kilometers of ice, significantly increasing the ability to capture these rare events, and possibly start zeroing in on the processes that generate them.
If IceCube has a hard time pinning down high energy neutrinos (at least until there's a nearby supernova; see sidebar), pity cosmologists. Just like the Cosmic Microwave Background (CMB) photons that tell us about the Big Bang, there's a cosmic neutrino background created by the event itself that could tell us even more. And it consists of copious numbers of neutrinos; according to Fermilab's Bradford Benson, at the time the CMB was emitted, 10 percent of the Universe's energy density was neutrinos. Even today, despite their phenomenally light mass, "at the low end of the known [mass] range, neutrinos weigh as much as all the stars in the Universe," said Benson.
But, at such low energies (they're on the scale of a Mega-electronVolt), we have no way of possibly detecting the cosmic neutrino background. Until that changes, the CMB can tell us some things about neutrinos themselves—things that are difficult to determine because the particles are so annoying to work with. Benson works on the South Pole Telescope, located near IceCube, which examines a patch of the CMB in the southern skies, achieving a 13-fold boost over the space-based WMAP probe.
With these observatories, you can spot the acoustic oscillations of matter, caused by the counteracting pull of gravity and push of radiation pressure. And these tell us about the contents of the Universe itself; matching their properties is one of the great successes of the lambda-cold dark matter model. Referring to the model, Benson said "the CMB is the best piece of evidence that we live in this Universe." And this Universe contains a lot of neutrinos.
In fact, differences in neutrino masses of as little as 0.1 electronVolts is enough to change the amount of structure in the Universe (galaxy clusters and the like) by about five percent. Of course, it's possible that this value is more than half the combined mass of all three neutrino types, so it's not as informative as it might be. Still, the CMB places some of the tightest limits on the masses of neutrinos that we've identified.
It also places limits on the number of neutrinos. Right now, we know of three, but particles called "sterile neutrinos" have maintained a persistent presence on the theoretical scene. But Benson said that the latest analyses of the CMB produce a neutrino count of 3.15—close enough to the answer we already have that there's unlikely to be any surprises here. And, when it comes to neutrinos, a lack of surprises is a rare event.