Some of the earliest and most successful neutrino detectors were based on enormous tanks of water. For example, Japan's Super Kamiokande held 3,000 tons of water, and researchers used the detector to watch for a sign that a neutrino had bumped up against one of the water molecules. A recently constructed detector takes a similar approach, observing about a cubic kilometer of water using over 5,000 optical sensors. It just relies on nature to provide the water. The detector is called IceCube, and its detectors are buried in the South Pole's ice cap.
IceCube has now scored its first big success, detecting the highest-energy neutrinos ever spotted. Odds are good that these neutrinos originated from an event distant from Earth, but remaining uncertainties mean that we can't conclude that with certainty.
The reason so much water is needed is that neutrinos don't like to interact with normal matter. Each second, a trillion neutrinos pass through your hand, but only about two will interact with an atom in your body throughout your entire lifetime. Spotting a neutrino requires a detector with a lot of material. Water has worked well, simply because it's relatively easy to get lots of it into one place and because it's transparent to much of the light that's created when high-energy neutrinos collide with an atom. Simply point enough photodetectors at a big tank of water and wait.
But rather than building a tank, the IceCube team decided to go where the water was already. They dropped strings of photodetectors to depths of over 2km in holes drilled in the ice and started watching for the tell-tale light. Since the bandwidth from the detectors to the rest of the world is pretty low, they even set up a server farm at the South Pole to filter the results for interesting events first.
These events are caused when high-energy neutrinos interact with an atom and create a spray of particles, some of which take away enough energy that they would move faster than the speed of light in the ice. To slow down, they emit detectable Cherenkov radiation.
Most of the higher energy neutrinos that IceCube has detected have come from collisions between cosmic rays and our atmosphere. However, there are limits to how energetic these neutrinos will get, and the IceCube team searched above those limits by adding up the energy in all of the photons that were detected as a single event.
Earlier this year, scientists using a powerful detector at the South Pole discovered Ernie and Bert, two neutrinos with energies over 100 times higher than the protons that circulate in the LHC. Now, the same team has combed through its data to find an additional 26 high-energy events, and they've done a careful analysis to show that these are almost certainly originating from somewhere outside our Solar System.
Neutrinos are incredibly light particles that rarely interact with normal matter; staggering numbers pass through the Earth (and your body) every second. To spot one, you need a very large detector, and IceCube fits the bill. Located in the ice cap at the South Pole, the detector works by capturing the light produced when neutrinos interact with the huge volume of ice present. To do so, holes were drilled up to 2 km into the ice, and strings of photodetectors were lowered into them. All told, they pick up the signals from a cubic kilometer of ice.