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BU physicists challenge theory of universe
By Hope Green
Creating a stir in the international physics community, BU scientists recently helped to uncover evidence that challenges a widely held framework for understanding the universe.
The theory in question, known as the Standard Model of Particle Physics, has for the past 30 years been used to explain how the building blocks of matter -- such as protons, electrons, and neutrons -- are arranged and to describe the forces by which they interact. The Standard Model underlies many theories of how the cosmos was formed and what holds matter together.
But preliminary results of a decade-long experiment, conducted at the U.S. Department of Energy's Brookhaven National Laboratory on Long Island, N.Y., appear to contradict the assumptions in this model for the first time.
"This work could open up a whole new world of exploration for physicists," says Lee Roberts, CAS physics professor and cospokesperson for the experiment. "The Standard Model has been successful and has explained many things that once were a puzzle. But we have always understood that this theory is only an approximation of what the real world is. We've known for some years that there must be something beyond it."
One area of research that could benefit from the experiment is the investigation of supersymmetry, a theory that predicts a new family of subatomic particles.
"They are related to the particles we know, but nobody's ever seen them," Roberts says. "Scientists have predicted there is missing mass in the universe that we can't see, and some of that so-called dark matter could be these particles." Results of the Brookhaven study, he says, could be the first glimmer of evidence that such particles exist.
Several BU physicists have played a major role in the study at Brookhaven, collaborating with researchers from 10 other institutions in the United States, Russia, Japan, and Germany. They employed a powerful particle accelerator, a ring-shaped superconducting magnet, to observe the activity of heavy electrons known as muons.
The muon behaves like a tiny permanent magnet, similar to the permanent magnet in a compass needle, explains CAS Physics Professor James Miller, who led the analysis team for the experiment. The rate at which the compass rotates toward the North Pole is a measure of its magnetic moment, or how strong the magnet is.
"We measured the strength of a muon's magnetic moment by placing it in an external magnetic field and watching how fast it turns," Miller says. But unlike the compass needle, the muon is also spinning on its own axis like a toy top.
The scientists calculated the effects of three phenomena, known as the strong, weak, and electromagnetic forces, on that rate of spin as the muon moved through a magnetic field. Until now, all experiments have resulted in values that confirm what the Standard Model predicts.
But the Brookhaven group, engineering the most precise technology ever devised for such an investigation, came up with a measurement that deviates from the Standard Model.
"The muon is unique in that it is possible to both measure and predict its magnetic moment with great accuracy, using the Standard Model," Miller says. "The model itself seems to explain data from virtually all experiments to date, but it is in many ways an incomplete theory. Most physicists think that it is a stepping-stone to a more fundamental theory, and most major experiments in high-energy physics are trying to find cracks in the model. Therefore, it is very significant news if an experiment appears to disagree with a Standard Model prediction."
Close to discovery
The scientists caution that while their preliminary finding has a 99 percent chance of being accurate, it would be premature to call it a discovery. The international team of scientists and engineers, who started collecting data in 1997, still has a full year's worth of measurements to analyze. "If our new number agrees with our current number," Roberts says, "then our finding will be at the discovery level."
Results of the experiment have been submitted to Physical Review Letters.
Boston University physicists have been key players in the Brookhaven study from its inception. Roberts and Miller began participating in 1984 at the urging of Lawrence Sulak, who was then developing the Brookhaven proposal while on a sabbatical visit to Boston. He stayed on to become chairman of the CAS physics department.
William Worstell, a former assistant professor, joined the project in 1986 and went on to found a BU startup company as a result. His firm uses detector technology developed for the muon study in high-resolution breast cancer scanning. Robert Carey, an assistant professor, joined the project in 1991 and developed all the computer simulation packages for the experiment.
The surprising new finding about muons represents the second time in three years that BU scientists have challenged conventional theories of particle physics. In 1998, CAS physicists played a major role in the pathbreaking discovery that a particle known as a neutrino, which was assumed to be massless, indeed has the property of mass.
"Our outstanding undergraduate and graduate students, professors Carey, Miller, and Roberts, and the technical staff of our shops, directed by Leo Dumais and Eric Hazen, built the components for the largest and most uniform superconducting magnet in the world, analyzed the data, and now have brought home the kudos," Sulak says.
"With the discovery of neutrino mass three years ago, the BU physics labs have made seminal contributions to the two most acclaimed physics results of our times -- the first two indications of physics beyond the Standard Model. I take personal pride in having initiated both lines of research and in bringing both to Boston University."