A Columbia University physicist is building an underground vat that will hold 800 tons of mineral oil in an attempt to trap the elusive oscillating neutrino.
Janet Conrad, assistant professor of physics at Columbia, leads an experiment at Fermi National Accelerator Laboratory -Fermilab, outside Chicago, that will accelerate one kind of neutrino into a powerful beam, then fire that beam into the ultra-pure mineral oil in an effort to witness those neutrinos transform themselves into another kind. Such a transformation, called a "neutrino oscillation," would be proof positive that neutrinos have mass, early indications of which have already turned the physics world upside down.
Her work also travels outside the laboratory. She has taken research techniques used to observe neutrinos and translated them into classroom experiments for Columbia students, to demonstrate their utility to other fields. Medical research has been profoundly influenced by basic physics research, and some treatments for cancer were developed at particle accelerators, Professor Conrad points out. When neutrinos interact with the nucleus of a molecule of mineral oil, a tiny flash of light, called a "scintillation," is emitted; medical researchers also observe scintillations in studies of radioisotopes. Professor Conrad is designing a series of laboratory experiments to demonstrate basic science techniques to undergraduates and high school students.
She received the nation's top science award for young researchers at a White House ceremony Feb. 10 for her "original contributions to measuring neutrino mass and connecting the measurement techniques to applications in medicine to inspire undergraduate and K-12 students."
Neal Lane, director of the White House Office of Science and Technology Policy, presented the third annual Presidential Early Career Awards in Science and Engineering to 20 young researchers from across the nation. The awards are granted by the National Science Foundation and are worth $500,000 over five years. They were established by President Clinton in February 1996.
Professor Conrad expects to use the funding to help construct the Booster Neutrino Experiment, or BooNE, at Fermilab. She was also the recipient last summer of a Faculty Career Development Award and, in 1996, a Career Advancement Award, both from NSF and both to study neutrinos. "We're extremely grateful to NSF for their continued support of this important basic science, and hope to merit the trust they have placed in us," she said.
Enrico Fermi coined the term "neutrino"; it means "little neutral one" in Italian. Physicists have since determined that neutrinos come in three varieties, or generations, corresponding to the groupings of subatomic particles in the prevailing physics theory, the Standard Model: electron, tau and muon neutrinos. The BooNE collaboration, which includes about 40 physicists and physics students from 11 institutions, is attempting to show that neutrinos of one generation can change into neutrinos of another.
"Neutrino oscillations are closely tied to the question of neutrino mass," said Professor Conrad, who is one of two spokespersons, or leaders, of the BooNE collaboration. "If we do see that neutrinos can change types, then quantum mechanics says that they must have mass."
Since neutrinos are so pervasive and numerous -- any space the size of a clothes closet holds at least a billion of them -- the notion that they have mass has already shaken the physics world. Astrophysicists are revising their calculations of the total mass of the universe and their models of its evolution and eventual fate. Theoreticians are devising new schemes to relate fundamental particles and forces, and even the Standard Model is getting a facelift. There is also hope that finding a neutrino with mass will help resolve the "missing mass" question: the mass of the visible, luminous matter in the universe is nowhere near enough to account for the gravitational attraction astrophysicists observe among stars, galaxies and clumps of galaxies.
Because neutrinos do not interact very often, physicists who study them need large detectors and an intense neutrino beam to see them. BooNE, approved by Fermilab last spring and set to run in the year 2001, is a spherical underground tank 40 feet in diameter holding 800 tons of ultra-pure mineral oil, similar to the baby oil sold in drugstores. In extremely rare events, a neutrino will interact with a molecule of mineral oil; when it does so, it emits a tiny flash of light. The tank will be lined with approximately 1500 phototubes, photon-sensitive detectors that will observe the photons emitted by neutrino interactions.
The neutrino beam is generated in the Booster, one of Fermilab's underground particle accelerators. It will accelerate a very pure beam of muon neutrinos and direct them into the vat of mineral oil. If scientists observe a significant number of electron neutrinos in the oil, they will know that oscillations have taken place.
Dr. Conrad's investigations are not taking place in a vacuum. Several experiments already have offered researchers clues that neutrinos have mass, however slight or imperceptible. Most notably, researchers at the Super-Kamiokande experiment west of Tokyo reported in June that they observed extremely rare interactions between neutrinos and atomic nuclei, from which they inferred the mass of a neutrino, in an underground tank containing 50,000 tons of ultra-pure water. Other indications of neutrino oscillations have come from the Homestake Experiment, a cavern filled with dry-cleaning fluid in the former Homestake gold mine in South Dakota. And the Liquid Scintillator Neutrino Detector, or LSND, experiment at Los Alamos National Laboratory in New Mexico has seen about 50 interactions that may indicate muon neutrino to electron neutrino oscillations.
To verify that what LSND investigators observed are really oscillations, Professor Conrad and her collaborators need to find many more of these interactions under different experimental conditions. If the effect continues to occur, then it will be clear that the LSND result is really the result of oscillations. The BooNE collaborators will conduct experiments designed to conclusively verify or disprove the LSND result.
"Three different experiments have now seen indications for oscillations," Professor Conrad said. "But in every case, very few events have been seen, so it is hard to be sure."
The program of experiments to be run on BooNE was developed by Professor Conrad and by Michael Shaevitz, professor of physics at Columbia. The research group also includes five Columbia graduate students and four undergraduates.
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