MiniBooNE Data Clarifies Neutrino Behavior

The initial data from the 10-year-long “MiniBooNE” experiment at the Department of Energy's Fermilab significantly clarifies the overall picture of how the fundamental particles, neutrinos, behave and provides staunch new evidence for the idea that only three low-mass neutrino species exist. These results, reported over the past week at a Fermilab lecture and at the American Physical Society (APS) meeting in Jacksonville, Fla., seem to rule out two-way neutrino oscillations involving a hypothetical fourth type of low-mass neutrino.

A neutrino signal observed by the MiniBooNE experiment. (Photo courtesy Fermilab)

The project was designed to confirm or refute surprising observations from the Department of Energy's Los Alamos National Laboratory’s Liquid Scintillator Neutrino Detector (LSND) experiments in the 1990s that were explained simply by the ability of neutrinos to transform from one type into another and back again, a process called neutrino oscillation. This research showed conclusively that there is more to the story, said Yale Assistant Professor Bonnie Fleming, a MiniBoone project participant who announced the results locally last week at the university.

The MiniBooNE experiment mimicked the earlier Liquid Scintillator Neutrino Detector (LSND) experiment by looking for signs of muon neutrinos oscillating into electron neutrinos in the region indicated by the LSND 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 simple LSND oscillation interpretation.

The MiniBooNE experiment relies on a 250,000-gallon tank filled with mineral oil, which is clearer than water from a faucet. Light-sensitive devices (PMTs) mounted inside the tank are capable of detecting collisions between neutrinos and carbon nuclei of oil molecules. (Photo courtesy Fermilab)

The MiniBooNE (short for Booster Neutrino Experiment; the "mini" refers to the fact that they use one detector rather than the originally proposed two) team is an international collaboration of 77 researchers from 17 institutions in the US and the UK working at Fermilab, with support from the Department of Energy and the National Science Foundation.

MiniBooNE co-spokesperson Janet Conrad of Columbia University said, “Our results are the culmination of many years of very careful and thorough analysis. This was really an extraordinary team effort. We know that scientists everywhere have been eagerly waiting for our results.”

Neutrinos were first theorized to account for energy that seemed be to be “missing” after nuclear beta decay. But, according to Fleming, “Although they are fundamental building blocks of the universe, they have been very difficult to detect and measure.”

At present, 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.

Bonnie Fleming with a photoreceptor from the MiniBooNE experiment. (Photo courtesy Yale)

“You can’t see them, hear them or touch them, but neutrinos are everywhere," Fleming said. "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,” she said. “And they are produced in many ways -- by the sun or when stars explode, or by us using particle accelerators. So, it is important for us to understand their nature and how they behave.”

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, the LSND findings cried out for independent verification.

“It was very important to verify or refute the surprising LSND result,” said Robin Staffin, DOE associate director of science for high-energy physics.

For its observations, MiniBooNE relies on a detector made of a 250,000-gallon tank filled with ultrapure mineral oil, clearer than water from a faucet. A layer of 1280 light-sensitive photomultiplier tubes, mounted inside the tank, detects collisions between neutrinos made by the Booster accelerator and carbon nuclei of oil molecules inside the detector.

A part of Fleming’s earlier graduate work was on creation of the photoreceptor array for the experiment. Since January 2006, the MiniBooNE experiment has been collecting data using beams of antineutrinos instead of neutrinos and expects further results from these new data.

Although the MiniBooNE researchers have decisively ruled out the interpretation of the LSND results as being due to oscillations between two types of neutrinos, there are data they observe at low energy that do not match their expectations. At this time, the source of the apparent low energy discrepancy is unknown.

“It is great to get the MiniBooNE results out,” said Fermilab Director Pier Oddone. “It clears one mystery, but it leaves us with a puzzle that is important to understand.”

The MiniBooNE collaboration will further analyze its data.

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.

For more information, visit: www-boone.fnal.gov/about/nusmatter/