Particle Sleuths, continued
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.
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. Thats 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.)
scientists find the unassuming neutrino to be so compelling? In large
part, the particles 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 suns 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 doesnt
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 matterand a strange one at thatneutrinos could
also provide the answer to a key question: which of the solutions to the
superstring theoriestheories which seek to unify all the known forces
in the universeis 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, its 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.
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.
played a major role in SNO, from the detectors design, construction,
and operation to the data analysis. The first experiment to confirm the
existence of solar neutrinosdone in another mine, essentially using a
huge vat of dry-cleaning fluid to trap the particleswas conducted 33
years ago by Dr. Raymond Davis Jr., who would later join Penns faculty.
During the years between the two experiments, a host of Penn scientists
have contributed to solar neutrino research. I would say weve been more
involved than any other institution on this issue, says Beier.