New Form of Matter or Energy Suspected in Neutrino Trial
By Shankar Vedantam
Washington Post Staff Writer
Friday, November 9, 2001; Page A02
An experiment that involved smashing together certain subatomic particles
at great speeds produced an unexpected result, prompting physicists
yesterday to announce that they might be on the verge of finding a new
form of matter or energy.
Discrepancies between experimental and theoretical results have
previously led to the discovery of new particles or forces. While this
discrepancy could be a fluke, the scientists who conducted the experiment
said the odds were it represented something meaningful.
If some hidden matter or energy did cause the discrepancy, it may help
scientists find their way to the ultimate quest of physics -- a unified
theory that explains how all the particles and forces in the universe
interact with each other.
"It could be a very big deal," said Kevin McFarland, an assistant
professor in the Department of Physics and Astronomy at the University of
Rochester in New York who helped conduct the research. "It would be very
exciting if we find another force."
The scientists, who are associated with the Department of Energy's Fermi
National Accelerator Laboratory near Chicago, are conducting follow-up
experiments with a high-energy particle accelerator to verify and explain
the result.
The experiment involved a type of subatomic particle called a neutrino.
This particle is a cousin of the electron, which is well-known from high
school textbooks as one of the constituents of atoms. Unlike electrons,
which are negatively charged, neutrinos have no charge. Because they are
also very light compared with other subatomic particles, they behave in a
ghostly way -- wandering through the universe at the speed of light
without interacting very much with other particles.
"If you lie out in the sun, there are a hundred trillion neutrinos
passing through your body at any time," said McFarland.
In the experiment, researchers fired neutrinos at a target and calculated
how often the particles slammed into the nucleus of an atom. Because
neutrinos are "invisible" to scientists' detectors, the physicists
measured how often the collisions produced another type of particle,
called a muon. "It's like you're firing a gun into an apple," said
McFarland. "The bullet goes in and you get a spray of apple out the
back."
Since neutrinos are so small, most of the time they passed through the
nucleus without affecting it. The frequency of collisions told scientists
about the electromagnetic forces that affect how neutrinos behave -- the
so-called weak forces. The scientists found slightly fewer interactions
with one of the weak forces than had been predicted by the Standard
Model, physicists' current description of fundamental forces and
particles. Since the model is very precise, scientists concluded that the
difference was significant.
"This measurement differs from what we expect by three units," said
Michael Shaevitz, professor of physics at Columbia University and another
investigator. "On a statistical basis, that would be a 1 in 400
probability of happening as a result of chance. It would be way out there
on the tail of the probability curve."
Peter Meyers, a professor of physics at Princeton University who was not
part of the research team, said the finding is the "sort of crack" that
"has been sought for many, many years."
Meyers cautioned that he couldn't say whether the new finding amounted to
a significant discrepancy because he hadn't seen the actual data. But he
added that "the Standard Model started falling into place in the '70s.
People figured it wouldn't be the last word, but it's been 20-30 years
without finding discrepancies that have lasted very long."
McFarland said he thinks "there is a high probability that something is
wrong with the theory."
The research has been submitted for publication in the journal Physical
Review Letters. McFarland and Shaevitz's collaborating scientists were
from the University of Cincinnati, Kansas State University, Northwestern
University, the University of Oregon and the University of Pittsburgh.
