Issue of
June 4, 1997

Elusive neutrino raises key question
in particle physics

BY DAVID F. SALISBURY

Four universities have joined forces on a project designed to help answer one of the fundamental questions in particle physics: Does the neutrino have any mass?

Related Information:

• Professor Gratta's home page
  
  
• Palo Verde Neutrino Oscillation Project home page

Neutrinos are the most elusive of fundamental particles and are produced during natural radioactive decay. They have been dubbed "ghost particles" because they can travel for millions of miles through solid lead without being stopped. Large numbers of neutrinos were created during the primordial Big Bang, so they are commonplace throughout the universe. Scientists know that neutrinos have very little, if any, mass. But there are so many of these particles (some 300 per cubic centimeter) that if they weigh anything at all they could account for the "missing mass" in the universe. If they do have mass, the fact would modify our understanding of the basic laws of physics as well as have important consequences in astrophysics and cosmology.

On May 9, officials from Stanford, the California Institute of Technology, the University of Alabama and Arizona State University gathered at the Palo Verde Nuclear Generating Station, 60 miles from Phoenix, to inaugurate a new neutrino laboratory. The Palo Verde Neutrino Oscillation Project Project has been constructed to conduct a $2 million, two-year research effort. Construction of the new facility was begun last spring and will be completed this summer. The purpose of the project, which will be conducted by Stanford Associate Physics Professor Giorgio Gratta and Professor Emeritus Felix Boehm from Caltech, is to use the neutrinos produced by the nearby nuclear reactors at the Palo Verde Generating Station to shed new light on the neutrino mass question.

If it exists, the neutrino's mass is too small to measure directly. So the project will be looking for an indirect indication that relies on the fact that neutrinos come in three slightly different varieties. If they have any mass at all, theory predicts that individual neutrinos should cycle among the three different forms at a rate that depends on the minute differences in their masses. The facility is located slightly more than a half mile from the reactors, far enough so that a certain percentage of the reactor neutrinos should have time to switch from the type of neutrino that the underground detector - which consists of 12 tons of liquid surrounded by 300 tons of shielding - was designed to detect to a type that will pass through it. If the neutrinos are not changing form, then the scientists calculate that they should be able to detect about 50 particles a day coming from the reactors. If they measure significantly fewer than that, it will indicate that the reactor neutrinos are cycling to other forms and, therefore, have mass.

In addition to the Stanford research group - Gratta, research associate Yi-Fang Wang, and graduate students Lester Miller and Dillon Tracy - the campus was represented at the inauguration by associate dean Hans Andersen and financial director Nancy Padgett from the School of Humanities and Sciences, and chairman Blas Cabrera and manager Rosenna Yau from the Department of Physics. They were joined by officials from the Arizona Public Service Company, which operates the nuclear generating plant, and the Department of Energy, as well as administrators and scientists from Caltech, the University of Alabama and Arizona State University. SR