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The Neutrino Frontier

At fermilab, particle smashing yields to flavor changing
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FERMILAB
Since 1983 researchers at the Fermi National Accelerator Laboratory in Batavia, Ill., have plumbed the subatomic realm by smashing high-energy protons and antiprotons together in the Tevatron, the world's most powerful particle collider. Next year, however, the high-energy frontier will move to Europe, where the even more powerful Large Hadron Collider will begin operations near Geneva. Fermilab intends to shut down the Tevatron by 2010. But rather than scrapping the device, lab officials have outlined an ambitious plan to use some of the collider's parts to enhance a promising research program: the study of the mysterious neutrino, whose strange properties may offer clues to new laws of physics.

Appropriately enough, the lab's name-sake--physicist Enrico Fermi--coined the name for the particle, which means "little neutral one." Neutrinos come in three types, called flavors; the most common are electron neutrinos, which are produced in copious amounts by reactions in the sun. (The other flavors are muon and tau neutrinos.) Because neutrinos have no charge and interact with other particles only through the weak nuclear force and gravity, they pass through matter virtual-ly unhindered and are devilishly difficult to detect. Until recently, most scientists thought neutrinos had no mass either, but in the late 1990s researchers found that the particles change flavor as they travel, and this trans-formation can happen only if they have mass.

Quantum theory predicts that these changes should be oscillatory, so physicists are now trying to measure how frequently neu-trinos change flavor and the likelihood of each transition. Whereas previous experiments passively observed neutrinos created in the sun or in the earth's atmosphere, Fermilab investigators decided to generate intense beams of neutrinos using the same accelerators that supply protons to the Tevatron. (The high-energy protons crash into beryllium targets, producing pions that spit out neutrinos as they decay.) In 2002 researchers began firing a beam of muon neutrinos at a giant spherical tank half a kilometer away. Called the MiniBooNE detector, the 12-meter-wide tank contains some 800 tons of mineral oil and 1,500 photomultipliers that recognize the rare flashes of light caused by neutrino inter-actions with the oil. The MiniBooNE team is currently analyzing three years of data to determine how many of the particles changed to electron neutrinos while in flight.

In 2005 investigators started the MINOS experiment, which shoots a more powerful beam of muon neutrinos across 735 kilometers to a massive detector in the abandoned Soudan Iron Mine in northern Minnesota. (The long journey gives the particles more time to oscillate.) This past March, MINOS scientists announced that only about half of the expected muon neutrinos arrived at the Soudan detector, suggesting that the remainder changed flavor along the way. The results were consistent with those of the earlier K2K experiment, which studied muon neutrinos fired from an accelerator at Japan's KEK laboratory.

Fermilab researchers are now designing an experiment called Minerva, which would improve the accuracy of the MINOS measure-ments by studying neutrino inter-actions with atomic nuclei. Scientists have also begun planning a study called NOvA, which would place another huge detector in northern Minnesota to hunt for electron neutrinos generated by the oscillations in the MINOS beam.

Because most neutrino detectors are designed to identify just one flavor, no single experiment can measure all the parameters of the oscillations. Fortunately, similar investigations in Japan may complement the U.S. studies. To increase their chances of success, Fermilab engineers are tweaking their acceler-ators to maximize the power of the MINOS beam, which in turn raises the number of neu-trino interactions in the detector. Once the Tevatron is shut down, the lab intends to boost beam power even further by reconfiguring facilities now used to produce and store anti-protons.

Scientists are fascinated by neutrino oscillations because they may reveal phenomena that cannot be explained by the Standard Model, the highly successful but incomplete theory of particle physics. For example, the MiniBooNE results may confirm the existence of a fourth kind of neutrino--the so-called sterile neutrino--that is not subject to the weak force but may participate in novel interactions that have not yet been identified. "The neutrino is one of the least understood particles," says Richard Van de Water of the MiniBooNE team. "If there's extra physics, it's a good place for nature to hide it."

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