standard_model_010208_MB_ An international team of researchers announced Thursday findings in the subatomic world that, if proven accurate, could upset a basic set of laws that scientists use to describe the physical world. The potential change in thinking would force cosmologists to reconsider the origin, evolution and daily operation of the universe.
The discovery involves the study tiny particles called muons. The data, generated at Brookhaven National Laboratory in New York, conflicts with previous measurements that explained the behavior of the subatomic particles.
Scientists consider muons the most basic of particles. They are so tiny that they aren't made of smaller particles, yet they play an outsized role in shaping the physical world.
| What |
| Unlike larger particles, muons don"t have any root physical structure -- they are not made out of any smaller building blocks. The presence of electric and other fields, however, does give muons some dimension. A proton, on the other hand, is made up of smaller things called quarks. The electron (which also has no root structure) is a stable particle, while the muon is radioactive and decays after some period of time. |
Brad Keister, director of the nuclear physics program at the National Science Foundation, said the finding, if it holds up, will change how cosmologists view the evolution of the universe back to and including the Big Bang, though this basic theory of how everything began would not necessarily be tossed out.
The work, supported in part by the NSF, will also put researchers on the trail of new theories to explain missing matter in the universe, Keister said. Scientists have only accounted for some 10 percent or less of the mass known to exist. The rest, noted by how its gravity affects visible objects, is sometimes called dark matter.
"It's a milestone," said Keister, who was not involved in the research. "If the result holds up, it's exciting simply because it says, 'There's more out there.'"
Tiny world with huge implications
A muon is a subatomic particle, something like an electron but heavier. The muon and many other small particles (with names like "quark" and "tau") have been discovered over the past several decades as researchers have developed ever more powerful particle accelerators used to study the goings-on of the subatomic world.
The Standard Model of particle physics, which has been in development since the 1960s, explains the behavior of these particles. The model predicts, among other things, how a tiny muon should be affected as it moves through a magnetic field.
In particular, the Standard Model uses a thing called a g-2 value (pronounced "g minus 2") to measure the effect of three primary forces of nature on the muon.
The three forces are known as the weak force (which describes radioactive decay), the strong force (which binds the nuclei of particles together), and the electromagnetic force (which runs lights and allows radio communication). These forces alter a characteristic of muons known as "spin," which is somewhat similar to the spin of a toy top.
Gravity, the fourth known force in the universe, is not considered. This shortcoming in the Standard Model is something scientists would like to rectify with a larger, unified model of the physical world, says Morris Aizenman of the NSF's Mathematical and Physical Science Directorate.
"The Standard Model has worked extraordinarily well for over 30 years in describing three of the forces," Aizenman says. "It never attempted to combine the force of gravity with the other three."
Previous measurements of this g-2 value agreed with the Standard Model. But the new experiments, using a very intense source of muons and the world's largest superconducting magnet, has yielded what researchers expect are more precise results. And they don't agree with the Standard Model (which incorporates Einstein's theory of relativity).
"We are now 99-percent sure that the present Standard Model calculations cannot describe our data," said Brookhaven physicist Gerry Bunce, project manager for the experiment.
The finding could mean that new and strange types of physics might be possible. One idea that could benefit from the possible upheaval in thinking is called supersymmetry, a theory that predicts the existence of companion particles for all the known particles in the universe.
"Many people believe that the discovery of supersymmetry may be just around the corner," said Boston University physicist Lee Roberts, who also worked on the project. "We may have opened the first tiny window to that world."
These companion particles, while never before seen, would not surprise most theorists, who have long suspected that "empty" space is actually a sea of virtual particles that appear and disappear almost instantaneously. These particles, if found to exist, might account for some or all of the missing mass in the universe.
The behavior of the muons in the Brookhaven study seem to fit this idea, the researchers said. New studies with more powerful particle accelerators, two of which are planned over the next four years, would be needed to produce these companion particles, if they exist.
The scientists involved in the study -- 68 researchers from 11 institutions in the United States, Russia, Japan and Germany -- agree that more research is needed to confirm the work.
But further work by the group -- there is a year's worth of data still to analyze -- won't say what is out there, points out Keister of the NSF. It would confirm that, as cosmologists have suspected, there is more to the universe than currently meets the eye.
Next page: Details of how the study was done
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