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Friday, September 12, 2008

Fermilab Looks for Visitors from Another Dimension

By Mark Alpert

Neutrino Hunters Bonnie Fleming and Mitchell Soderberg inspect a prototype liquid-argon detector called ArgoNeuT that will pave the way for the MicroBooNE facility at Fermilab.

The detection of extra dimensions beyond the familiar four—the three dimensions of space and one of time—would be among the most earth-shattering discoveries in the history of physics. Now scientists at the Fermi National Accelerator Laboratory in Batavia, Ill., are designing a new experiment that would investigate tantalizing hints that extra dimensions may indeed exist.

Last year researchers involved in Fermilab’s MiniBooNE study, which detects elusive subatomic particles called neutrinos, announced that they had found a surprising anomaly. Neutrinos, which have no charge and very little mass, form out of nuclear reactions and particle decays. They come in three types, called flavors—electron, muon and tau—and oscillate wildly from one flavor to another as they travel along. While observing a beam of muon neutrinos generated by one of Fermilab’s particle accelerators, the MiniBooNE researchers found that an unexpectedly high number of the particles in the low-energy range (below 475 million electron volts) had transformed into electron neutrinos. After a year of analysis, the investigators have failed to come up with a conventional explanation for this so-called low-energy excess. The mystery has focused attention on an intriguing and very unconventional hypothesis: a fourth kind of neutrino may be bouncing in and out of extra dimensions.

String theorists, who seek to unify the laws of gravity with those of quantum mechanics, have long predicted the existence of extra dimensions. Some physicists have proposed that nearly all the particles in our universe may be confined to a four-dimensional “brane” embedded within a 10-dimensional “bulk.” But a putative particle called the sterile neutrino, which interacts with other particles only through gravity, would be able to travel in and out of the brane, taking shortcuts through the extra dimensions. In 2005 Heinrich Päs, now at the University of Dortmund in Germany, Sandip Pakvasa of the University of Hawaii and Thomas J. Weiler of Vanderbilt University predicted that the extradimensional peregrinations of sterile neutrinos would increase the probability of flavor oscillations at low energies—exactly the result found at MiniBooNE two years later.

Energized by the prospect of discovering new laws of physics, the MiniBooNE team soon proposed a follow-up experiment called MicroBooNE that could test the sterile neutrino hypothesis. The new detector, a cryogenic tank filled with 170 tons of liquid argon, would be able to detect low-energy particles with much greater precision than its predecessor could. A particle emerging from a neutrino interaction would ionize the argon atoms in its path, inducing currents in arrays of wires at the perimeter of the tank. Scientists could then pinpoint the trajectory of the particle, allowing them to better distinguish between electron neutrino interactions and other events and thus determine whether there really is an excess of oscillations at low energies.

Estimated to cost about $15 million, the MicroBooNE tank would be located near the MiniBooNE detector at Fermilab so that it could observe the same beam of neutrinos. This past June the lab’s physics advisory committee approved the design phase for the project; if all goes well, the detector could begin operating as soon as 2011.

Researchers hope that MicroBooNE will lead to the development of much larger detectors, containing hundreds of thousands of tons of liquid argon in tanks as big as sports arenas. Such facilities could search for other hypothesized phenomena such as the extremely rare decay of protons. “It’s a fantastic new technology,” says Bonnie Fleming, a physicist at Yale University and spokesperson for MicroBooNE. “And it’s crucial for taking the next step in physics.”

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