Looking for Neutrinos, Nature’s Ghost Particles

To study some of the most elusive particles, physicists have built detectors in abandoned mines, tunnels and Antarctic ice

The cavernous Super-Kamiokande detector in Japan is lined with 13,000 sensors to pinpoint signs of neutrinos. (Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo)
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But the Homestake and Super-K experiments didn’t detect as many neutrinos as physicists expected. Research at the Sudbury Neutrino Observatory (SNO, pronounced “snow”) determined why. Installed in a 6,800-foot-deep nickel mine in Ontario, SNO contains 1,100 tons of “heavy water,” which has an unusual form of hydrogen that reacts relatively easily with neutrinos. The fluid is in a tank suspended inside a huge acrylic ball that is itself held inside a geodesic superstructure, which absorbs vibrations and on which are hung 9,456 light sensors—the whole thing looking like a 30-foot-tall Christmas tree ornament.

Scientists working at SNO discovered in 2001 that a neutrino can spontaneously switch among three different identities—or as physicists say, it oscillates among three flavors. The discovery had startling implications. For one thing, it showed that previous experiments had detected far fewer neutrinos than predicted because the instruments were tuned to just one neutrino flavor—the kind that creates an electron—and were missing the ones that switched. For another, the finding toppled physicists’ belief that a neutrino, like a photon, has no mass. (Oscillating among flavors is something that only particles with mass are able to do.)

How much mass do neutrinos have? To find out, physicists are building KATRIN—the Karlsruhe Tritium Neutrino Experiment. KATRIN’s business end boasts a 200-ton device called a spectrometer that will measure the mass of atoms before and after they decay radioactively—thereby revealing how much mass the neutrino carries off. Technicians built the spectrometer about 250 miles from Karls­ruhe, Germany, where the experiment will operate; the device was too large for the region’s narrow roads, so it was put on a boat on the Danube River and floated past Vienna, Budapest and Belgrade, into the Black Sea, through the Aegean and the Mediterranean, around Spain, through the English Channel, to Rotterdam and into the Rhine, then south to the river port of Leopoldshafen, Germany. There it was offloaded onto a truck and squeaked through town to its destination, two months and 5,600 miles later. It is scheduled to start collecting data in 2012.

Physicists and astronomers interested in the information that neutrinos from outer space might carry about supernovas or colliding galaxies have set up neutrino “telescopes.” One, called IceCube, is inside an ice field in Antarctica. When completed, in 2011, it will consist of more than 5,000 blue-light sensors (see diagram above). The sensors are aimed not at the sky, as you might expect, but toward the ground, to detect neutrinos from the sun and outer space that are coming through the planet from the north. The earth blocks cosmic rays, but most neutrinos zip through the 8,000-mile-wide planet as if it weren’t there.

A long-distance neutrino experiment is taking place under several Midwestern states. A high-energy accelerator, which generates subatomic particles, shoots beams of neutrinos and related particles as much as six miles deep, beneath northern Illinois, across Wisconsin and into Minnesota. The particles start at Fermilab, as part of an experiment called the Main Injector Neutrino Oscillation Search (MINOS). In less than three-thousandths of a second, they hit a detector in the Soudan iron mine, 450 miles away. The data the scientists have gathered complicates their picture of this infinitesimal world: it now appears that exotic forms of neutrinos, so-called anti-neutrinos, may not follow the same rules of oscillation as other neutrinos.

“What’s cool,” says Conrad, “is that it’s not what we expected.”

When it comes to neutrinos, very little is.

Ann Finkbeiner’s latest book, A Grand and Bold Thing, is about the Sloan Digital Sky Survey, an effort to map the universe.


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