Probing high-energy physics inside an atom.

High-energy particle accelerators are not the only tools available to physicists to test their theories of the fundamental particles and forces of nature. Lasers also help them study subtle details of the way electrons behave in certain types of atoms.

Such tabletop experiments probe the elementary particle physics frontier, says E. Norval Fortson of the University of Washington in Seattle.

Precision measurements on an atomic scale can test various aspects of the standard model of particle physics, which describes the interactions of quarks, electrons, and other components of matter, he notes. They may even provide glimpses of new physics beyond the standard model.

One such test involves the weak force, one of the particle interactions in the standard model. This force is responsible for radioactivity and plays a central role in the decay of an isolated neutron into a proton, electron, and antineutrino.

According to the standard model, the weak force acts on particles in a way that distinguishes left from right. This effect is known as parity violation.

For example, when a cobalt-60 nucleus having a particular orientation decays to create a nickel-60 nucleus, an electron, and an antineutrino, the electron is ejected preferentially in one direction. The existence of this favored direction in weak interactions makes the universe left-handed.

Fortson, Washington colleague Paul A. Vetter, and their coworkers looked for parity violation in the behavior of the outermost electron of a thallium  atom. The researchers carefully observed what happens when polarized laser light passes through thallium vapor.

A slight rotation in the angle of polarization provides a direct measure of parity violation in the interaction between an electron and the quarks of the thallium nucleus. Combining the experimental measurements with theoretical calculations of the distribution of electrons in a thallium atom, the Washington team obtained a result in good agreement with the standard model.

Fortson and Vetter reported their findings at last month's American Physical Society meeting, held in Washington, D.C.

Using a similar strategy but working independently, N.H. Edwards of the Clarendon Laboratory in Oxford, England, and his collaborators have made comparable measurements of parity nonconservation in thallium.

Certain aspects of the results hint that "there may be new physics here, but at this point, one can't say," Fortson says. "We need to do better."

By refining the electron distribution calculations and working with a single atomic ion rather than a vapor, the researchers hope to approach the precision achieved in particle accelerators.

At the same time, M. Scott Dewey and Geoffrey Greene of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., and their collaborators are investigating the details of parity violation by making precise measurements of the neutron lifetime. Such determinations offer additional insights into the nature of the weak interaction.

The researchers use neutrons generated in a research reactor at NIST. Slowed down, these neutrons are guided to a magnetic trap. If the neutron happens to decay, the resulting proton gets caged by the trap's magnetic field. An electric field around the trap then nudges the proton to a detector.

The NIST team expects to contribute a new value to the world average for the neutron lifetime -- which currently stands at 887 seconds -- within the next few months.