Icecube: In the Antartic

        The IceCube Neutrino Observatory is a massive neutrino detector in Antarctica that takes advantage of the fact that the south pole is covered in a medium through which charged particles can travel faster than light: ice. A kilometer down, the ice is beautifully clear, allowing bursts of Cherenkov radiation to propagate through it unhindered. The IceCube observatory itself consists of 5,160 digital optical modules, each about 25 centimeters in diameter, suspended on 86 individual strings lowered into boreholes in the ice. Each string sits between 1450m and 2450m below the surface, spaced 125m horizontally from neighboring strings, resulting in a neutrino detector that's a full cubic kilometer in size. What IceCube is looking for are the tiny flashes of blue light emitted by the electrons, muons, and tau particles flashing through the ice after a neutrino collides with a water molecule. These flashes are very dim, but there are no other sources of light that far down under the ice, and the photomultiplier tube inside each digital optical module can detect even just a handful of photons.
        Depending on what kind of subatomic particle the neutrino turns into, IceCube will detect different Cherenkov radiation patterns. An electron neutrino will produce an electromagnetic shower (or cascade) of particles. The muon produced by a muon neutrino, on the other hand, can travel hundreds of meters, leaving a track that points back along the same trajectory as the muon neutrino that created it. A tau neutrino will produce a sort of combination of these two signatures. Maybe. I think. Tau neutrinos are difficult to detect, because tau particles themselves are extraordinarily massive and short lived: they're something like 3,500 times the mass of an electron (and 17 times the mass of a muon), with a lifetime of just 0.0000000000003 second, which means that they decay into other subatomic particles virtually instantaneously and are easily mistaken for electron neutrinos. IceCube has some ideas of what unique radiation signatures might suggest the detection of a tau (including the "double bang," the "inverted lollipop," and the "sugardaddy"), but they haven't found one yet.

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