Yale Bulletin and Calendar

April 20, 2007|Volume 35, Number 26

By studying neutrino signals such as this, the researchers in the MiniBooNE experiment were able to show that an earlier interpretation of the small particles' behavior was incorrect.

Findings shed light on behavior of
fundamental particles called neutrinos

Yale physicist Bonnie Fleming was a participant in a project at the Department of Energy's (DOE) Fermilab that has clarified how fundamental particles called neutrinos behave.

The "MiniBooNE" project -- the first phase of the Booster Neutrino Experiment (BooNE) -- was undertaken to confirm or refute observations made during research on neutrinos in the 1990s at the DOE's Los Alamos National Laboratory.

The studies using Los Alamos' Liquid Scintillator Neutrino Detector (LSND) seemed to suggest that neutrinos had the ability to transform from one type into another and back again, a process called neutrino oscillation. Preliminary data from the MiniBooNE experiment conclusively show that this interpretation of the LSND findings is incorrect.

Theories about the existence of neutrinos were first put forth to account for energy that seemed be to be "missing" after nuclear beta decay, explains Fleming, assistant professor of physics.

"Although they are fundamental building blocks of the universe they have been very difficult to detect and measure," she says. "You can't see them, hear them, or touch them, but neutrinos are everywhere. They pass right by us and right through us. They can travel the distance of 200 Earths lined up before they 'hit' anything, and if you put your hand on the desktop and count to three, trillions will pass through it.

"And they are produced in many ways -- by the sun or when stars explode, or by us using particle accelerators," adds the Yale physicist. "So, it is important for us to understand their nature and how they behave."

Currently, three types, or "flavors," of neutrinos are known to exist: electron neutrinos, muon neutrinos and tau neutrinos. In the last 10 years, several experiments have shown that neutrinos can oscillate from one flavor to another and back. The observations made by the LSND collaboration also suggested the presence of neutrino oscillations, but in a neutrino mass region vastly different from other experiments.

Reconciling the LSND observations with the oscillation results of other neutrino experiments would have required the presence of a fourth, or "sterile," type of neutrino, with properties different from the three standard neutrinos. The existence of sterile neutrinos would throw serious doubt on the current structure of particle physics, known as the Standard Model of Particles and Forces.

Because of the far-reaching consequences of this interpretation, says Robin Staffin, associate director of science for high energy physics at the DOE. "It was very important to verify or refute the surprising LSND result."

Thus was launched the MiniBooNE experiment, an international collaboration of 77 researchers from 17 institutions in the United States and the United Kingdom working at Fermilab, with support from the DOE and the National Science Foundation.

The MiniBooNE researchers mimicked the earlier LSND experiment by looking for signs of muon neutrinos oscillating into electron neutrinos in the region indicated by the earlier observations. The team expected that the experiment would produce a distinct background and oscillation "signature."

The results found no appearance of electron neutrinos as predicted by a simple two-neutrino oscillation scenario, ruling out the LSND oscillation interpretation.

For the MiniBooNE experiment, researchers used a detector that includes a 250,000-gallon tank filled with ultrapure mineral oil, a liquid that is clearer than water from a faucet. A layer of 1,280 light-sensitive photomultiplier tubes mounted inside the tank detects collisions between neutrinos made by the booster accelerator and the carbon nuclei of the molecules of mineral oil. Fleming created the photoreceptor array used in the experiment while still a graduate student at Columbia University.

Although the MiniBooNE researchers have decisively ruled out the interpretation of the LSND results as being due to oscillations between two types of neutrinos, some of the data from their low-energy experiments do not match their expectations. At this time, the source of the apparent low-energy discrepancy is unknown.

Fermilab director Pier Oddone notes, "It clears one mystery but it leaves us with a puzzle that is important to understand."

The institutions participating in the MiniBooNE experiment included: University of Alabama, Bucknell University, University of Cincinnati, University of Colorado, Columbia University, Embry Riddle Aeronautical University, Fermi National Accelerator Laboratory, Imperial College-London (UK), Indiana University, Los Alamos National Laboratory, Louisiana State University, University of Michigan, Princeton University, Saint Mary's University of Minnesota, Virginia Polytechnic Institute and State University, Western Illinois University and Yale.

-- By Janet Rettig Emanuel

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