Neutrino expert to discuss insights, past and present

March 20, 2008 10:40 a.m.

Arthur McDonald, director of the Sudbury Neutrino Observatory in Ontario, Canada, will deliver the 33rd Donald Ross Hamilton Lecture at 8 p.m. Thursday, March 27, in A02 McDonnell Hall.

The lecture by McDonald, who is also the Gordon and Patricia Gray Chair in Particle Astrophysics at Queen's University in Kingston, Ontario, is titled, "A Deeper Understanding of the Universe From 2 km Underground." He is one of the world's leaders in neutrino astrophysics.

McDonald will describe his research, conducted over many years two kilometers below ground at the Sudbury Neutrino Observatory, which is attached to a fully functioning nickel mine. There, in an ultra-clean environment that is mostly free of the cosmic rays and radiation that can interfere with sensitive equipment, McDonald and his team have discovered that these unimaginably small neutrinos produced by the sun actually do have mass and can change type or "flavor."

"The electrons that make up the matter we know actually have two relatives, the muon and the tau meson, identical to the electron in all respects except that they are more massive (which is why we don't see them under normal conditions)," said Curtis Callan, chair of the Department of Physics, which is sponsoring the lecture. "Each of these particles is associated with a different neutrino: the neutrino emitted when a muon decays can't excite an electron and vice versa. We call this difference 'flavor,' just to give it a name."

The fact that there are three different neutrinos, he added, is important for cosmology: There is three times as much "neutrino pressure" contributing to the Big Bang expansion as there would be if there were only one neutrino "flavor." This effect recently was confirmed by the five-year data from the Wilkinson Microwave Anisotropy Probe satellite experiment led by Princeton scientists.

McDonald began paying attention to the elusive, mysterious particles in the 1980s. He was a Princeton faculty member from 1982 to 1989, when he became a professor at Queen's. While at Princeton, he helped found the Sudbury Neutrino Observatory, where under his leadership scientists would eventually solve the 30-year-old problem on neutrinos from the sun.

Neutrinos are elementary particles that travel close to the speed of light and pass through ordinary matter almost undisturbed. They are nearly impossible to detect. Most passing through Earth emanate from the sun.

In the late 1960s, scientists found that the number of neutrinos arriving from the sun was between a third and a half the number predicted by standard models. This discrepancy became known as the solar neutrino problem.

When experiments conducted by McDonald and his team at Sudbury showed that neutrinos both have mass and change type or "flavor," this resolved the neutrino puzzle. The neutrinos produced in the sun, they reasoned, had morphed into different flavors. This meant they could not have been detected by past experiments.

For his work, McDonald has won many awards, including being named last year as a co-winner of the Benjamin Franklin Medal for Physics as well as an Officer of the Order of Canada.

During the talk, McDonald will explain that there may be more discoveries to come. The observatory has been expanded to create a long-term, international facility for underground science -- called SNOLAB -- where measurements of dark matter, solar neutrinos and other mysterious physical entities are progressing.