Origins ANTARCTICA, Scientific Journeys from McMurdo to the Pole
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  © Per Olof Hulth
  An optical module hits the surface of the water. Below it is a column of water that will hold a string of these modules. The column will freeze, holding the modules in place until the Antarctic ice melts.

A literary essay about AMANDA by Francis Halzen
page 4

In principle, Super-K ought to detect equal numbers of low-energy neutrinos radiating in from all sides from the collisions of cosmic rays with the atmosphere. But in the past three years, the detector found that fewer neutrinos were coming in from the far side of the earth than from the near side. The only explanation for the discrepancy, the physicists concluded, was that some muon neutrinos, which Super-K can detect, must have changed into undetectable tau neutrinos as they passed through the earth.

The Super-K data are so precise, so elegantly conceived, that they seem to prove once and for all that neutrinos have mass—thereby punching a hole in one of the standard model’s tires. Whether fixing that hole will require a simple patch or a complete overhaul remains to be seen: Super-K can only measure the difference in mass between two neutrinos—a quantity somewhere between 0.1 and 0.01 electron volt. Based on that figure and other data, however, many physicists now estimate that the three types of neutrinos have a combined mass of around 0.1 electron volt.

What Super-K does not do—what no working detector has ever done—is pay attention to neutrinos from beyond our galaxy. Enter AMANDA. Because high-energy neutrinos are 1012 times easier to detect than solar neutrinos, our telescope can afford to trade sensitivity for size. The same volume of water surveyed by 13,000 photomultipliers in Super-K is surveyed by only around ten in AMANDA; our photomultipliers are less than half as large; and ours do not detect solar neutrinos at all. But AMANDA, when complete, will watch over thousands of times more water than Super-K. As a result, it will track neutrinos across as far as a kilometer, whereas Super-K can track them across no more than fifty meters.

When a high-energy neutrino collides with a proton in the ice, it creates a muon—a particle closely related to the electron but more than 200 times more massive—that continues along the neutrino’s upward path, streaming photons along its sides like a bottle rocket. The result is a hurtling cone of blue Cherenkov light—light of the same kind emitted by nuclear reactors. From the timing of the cone’s reception by our grid of photomultipliers, the muon’s direction can be reconstructed and the direction of the incoming neutrino inferred.

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