The Particle Sleuths, continued

From left: Penn grad student Doug McDonald, postdoctoral researcher Josh Klein and grad student Peter Wittich, and others with the acrylic vessel at the Sudbury Neutrino Observatory.

Mysterious Ways
Neutrinos have no charge, apparently very little mass, and are essentially the most weakly interacting of all subatomic particles. Though you would never notice it, every second some 60 billion neutrinos from the sun zip through a space roughly the size of the button on your sleeve. That’s not even counting the neutrinos that come from other sources, including the atmosphere and radioactivity within the earth. (They can also be manufactured by scientists in particle accelerators and nuclear reactors.)
    Why do scientists find the unassuming neutrino to be so compelling? In large part, the particle’s appeal lies in what it may be able to tell us about our own sun and other astronomical objects, as well as about the nature of matter itself.
    The nuclear reactions that power our sun, for instance, produce most of their energy in the form of photons of light, but our sun is so dense that a photon produced there takes about 10,000 years to diffuse to the sun’s surface, notes Paul Langacker, a Penn theorist who has not been directly involved with SNO. “By the time a photon has bounced around for 10,000 years, it doesn’t tell you much about the reactions that produced it at the center. But some of the energy is emitted as neutrinos, and they come right out. By observing solar neutrinos, one has a direct probe of what happens at the center of the sun.”
    As one of the basic building blocks of matter—and a strange one at that—neutrinos could also provide the answer to a key question: which of the solutions to the superstring theories—theories which seek to unify all the known forces in the universe—is actually correct?
    Though the idea of going more than a mile underground to search for particles that come from the sun may sound odd, even unsettling, it’s actually a necessary step to screen out cosmic radiation that rains down on us constantly and would otherwise produce too many distracting signals in a neutrino detector. Neutrinos, because they react so weakly with other matter, pass easily through the Earth.
    Scientists have long known that neutrinos come in three flavors, each named for the charged particle they are associated with: electron-neutrino, muon-neutrino, and tau-neutrino. What SNO has demonstrated is that they oscillate between these flavors on their path from the sun, and in order to do so, they likely have at least some mass.
    Penn researchers played a major role in SNO, from the detector’s design, construction, and operation to the data analysis. The first experiment to confirm the existence of solar neutrinos—done in another mine, essentially using a huge vat of dry-cleaning fluid to “trap” the particles—was conducted 33 years ago by Dr. Raymond Davis Jr., who would later join Penn’s faculty. During the years between the two experiments, a host of Penn scientists have contributed to solar neutrino research. “I would say we’ve been more involved than any other institution on this issue,” says Beier.

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