High-precision tests in particle physics.

High-precision tests in particle physics 

As high-energy particle accelerators grow more difficult and expensive to build and operate, and the collisions they generate become more complex to monitor and interpret, physicists are turning to alternative ways of probing the fundamental laws of matter. Such novel approaches, which rely on advanced lasers and other sophisticated instruments, are now yielding remarkably precise measurements of the properties of electrons and other particles. Moreover, these relatively inexpensive, "tabletop" experiments furnish extremely sensitive tests of theories predicting the behavior of atoms and subatomic particles. In many cases, their precision far surpasses that of present-day accelerators.

The contrast between the atom-smashing approach to physics and its alternatives is striking. Using a high-energy accelerator is like trying to figure out how a watch works by smashing it with a sledgehammer, then examining the fragments. On the other hand, instead of destroying the watch, researchers can try to deduce what's happening inside the watch simply by observing subtle vibrations of its casing. Both approaches are indirect, but the gentler techniques are proving superior for certain kinds of determinations. Such methods for making high-precision measurements were the subject of a session at this week's meeting in San Francisco of the American Association for the Advancement of Science and the American Physical Society.

To compare the properties of electrons and positrons (the antimatter equivalent of electrons), Hans G. Dehmelt and his colleagues at the University of Washington in Seattle isolate a single electron or positron in an electromagnetic trap, holding the same particle for hours or even days at a time. Such a trap, in which the particle is virtually stationary, recently allowed the researchers to show that electrons and positrons have the same magnetic moment to within a few parts in 10.sup.12.

"This work severely tests the fundamental theory of quantum electrodynamics and the mirror symmetry of electrons and positrons," Dehmelt says.

Similar experiments also indicate that the electron's radius must be less than 10.sup.-20 centimeters, less than one-thousandth the value of the previously accepted upper limit on the electron's radius (determined by smashing electrons together). In addition, by using traps holding individual, singly charged ions, researchers have managed to detect quantum jumps, in which an electron shifts from one energy level to another.

To test the "standard electroweak model," a remarkably successfull theory uniting electromagnetism and the weak nuclear interaction, researchers are using advanced laser technology to detect tiny distortions in heavy atoms such as cesium. Electrons in atoms "feel" not only the electromagnetic force between an electron and a positively charged nucleus, but also the much smaller influence due to the weak nuclear interaction. According to the standard electroweak theory, that additional effect distorts an atom about as much as a single hair added to Earth's surface changes the planet's shape. By measuring the distortion precisely, physicists can determine whether the strength of the weak nuclear force deviates at all from the strength predicted by the standard model.

To date, the most precise mesurements of this distortion come from Carl E. Wieman and his colleagues at the University of Colorado in Boulder. Using cesium atoms, they obtained values in agreement with the standard model and with previous, less precise measurements.

"While the results are now in excellent agreement with the standard model, it makes sense to continue these experiments, provided they can be done with sufficient precision in atoms amenable to unambiguous theoretical analysis," says Eugene D. Commins of the University of California, Berkeley. Such experiments demonstrated that the model holds precisely over a wide range of conditions. Furthermore, physicists hope to uncover the precise nature of subtle, previously unmeasurable contributions to the effects observed.