New Neutrino Experiment at Fermilab Goes Live

by Kurt Riesselmann

Scientists of the Booster Neutrino Experiment collaboration announced on September 9 that a new detector at the U.S. Department of Energy’s Fermi National Accelerator Laboratory has observed its first neutrino events. The BooNE scientists identified neutrinos that created ring-shaped flashes of light inside a 250,000-gallon detector filled with mineral oil.

The major goal of the MiniBooNE experiment, the first phase of the BooNE project, is either to confirm or refute startling experimental results reported by a group of scientists at the Los Alamos National Laboratory. In 1995, the Liquid Scintillator Neutrino Detector collaboration at Los Alamos stunned the particle physics community when it reported a few instances in which the antiparticle of a neutrino had presumably transformed into a different type of antineutrino, a process called neutrino oscillation.

“Today, there exist three very different independent experimental results that indicate neutrino oscillations,” said Janet Conrad, a physics professor at Columbia University and cospokesperson of the BooNE collaboration. “Confirming the LSND result would suggest the existence of an additional kind of neutrino beyond the three known types. It would require physicists to rewrite a large part of the theoretical framework called the Standard Model.”

Over the next two years, the BooNE collaboration will collect and analyze approximately one million particle events to study the quantum behavior of neutrinos. Neutrinos play an integral role in decay and fusion processes. The sun, for example, sends out an incredible amount of neutrinos, invisible to the naked eye. Although neutrinos are among the most abundant particles in the entire universe, little is known about the role of these ghost-like particles in nature.

“It is an exciting time for neutrino physics,” said Department of Energy Office of Science Director Raymond Orbach. “In the past few years experiments around the world have made extraordinary neutrino observations, shattering the long-standing view that neutrinos have no mass. The MiniBooNE experiment has the potential for advancing the revolution of our understanding of the building blocks of matter.”

Only in the last several years have scientists begun to shed light on the mysterious behavior of the three types of neutrinos – electron, muon and tau neutrino. Originally thought to be massless, experiments at the Superkamiokande neutrino detector in Japan have shown that neutrinos indeed have mass, allowing the particles to morph into each other. In 2001, experiments at the Sudbury Neutrino Observatory in Canada substantiated the Superkamiokande findings.

“MiniBooNEis an EXAMPLE of a SUCCESSFUL PARTNERSHIP among federal agencies, universities and national laboratories.”
—Marvin Goldberg, NSF program director

To simultaneously explain all experimental results, including LSND, introducing neutrino masses is not enough. Hence physicists have hypothesized the existence of a fourth type of neutrino, with properties rather different from the three types known so far. It could explain a range of neutrino-oscillation phenomena. Since the additional particle would interact with its surroundings even less than the three conventional neutrinos, scientists have named it the sterile neutrino.

The MiniBooNE experiment will now put the sterile-neutrino theory to the test. The experiment examines the behavior of an intense beam of muon neutrinos, created by the Booster accelerator at Fermilab. After traveling about 1,500 feet, the neutrino beam traverses the MiniBooNE detector. According to the LSND results, the distance is just right to allow a fraction of the muon neutrinos to transform into electron neutrinos.

The detector consists of a tank filled with ultraclean mineral oil, which is clearer than water from a faucet. The tank’s interior is lined with 1,520 light-sensitive devices, called photomultiplier tubes, which record tiny flashes of light produced by neutrinos colliding with carbon nuclei inside the oil. Based on the pattern and the timing of the light flashes, scientists can identify the type of neutrino that created a collision.

“We will operate the experiment 24 hours a day, seven days a week,” said Bill Louis, a Los Alamos scientist and cospokesperson of the BooNE collaboration. “We will be looking for oscillations of muon neutrinos into electron neutrinos. If nature behaves as LSND suggests, our detector will collect about one thousand electron neutrino events over the next two years. If not, we won’t see any excess of electron neutrinos. Either way, we’ll get a definite answer.”

The MiniBooNE experiment began taking data on August 24. Since then, the data acquisition system has been on-line 99.8 percent of the time. Two of the 66 BooNE scientists, who come from 13 institutions from across the United States, are monitoring the equipment around the clock.

“It’s not an issue to find people for the midnight shift,” said Bonnie Fleming, a Fermilab scientist working on MiniBooNE. “Now that we have beam, everybody is eager to do shifts, even at night.”

Construction of the MiniBooNE experiment lasted from October 1999 to May 2002. It required the construction of a 40-foot-diameter tank of steel surrounded by a concrete building. In addition, scientists had to build a beam line to transport protons from the Booster accelerator to a target building, in which the protons hit a metal block to produce muon neutrinos. The funding for the $19 million MiniBooNE experiment has come from the DOE’s Office of Science and the National Science Foundation.

“In addition to the importance of the science, MiniBooNE is an example of a successful partnership among federal agencies, universities and national laboratories,” said Marvin Goldberg, program director at NSF. “The project has also set new standards for education and public outreach in the field of high-energy physics. The small scale of the project allows undergraduate and graduate students to participate fully in all of the experimental components.”

ON THE COVER: Scientists of the Booster Neutrino Experiment collaboration announced on September 9 that a new detector at Fermilab has observed its first neutrino events. The BooNE scientists identified neutrinos that created ring-shaped flashes of light, here read out by a computer display, inside a 250,000-gallon detector filled with mineral oil.

On September 9, Fernanda G. Garcia, one of the young scientists of the collaboration, gave the MiniBooNE report at the weekly meeting of Fermilab experimenters. The highlight of her talk was the presentation of the first neutrino event observed by the detector, featuring a ring of light caused by a muon neutrino collision.

“We now have a small sample of neutrino events that we can study,” she said. “All forthcoming neutrino events we will collect in a ‘black box,’ making sure that we develop our analysis tools without knowing the exact content of the box. When we have collected enough events—in about two years—we will open the box and get our ultimate count of electron-neutrino events.”

Then, the BooNE collaboration will reveal the ending of an important chapter on the mysterious neutrinos. The whole story, however, will captivate scientists for decades to come.