The electron's other charge: workhorse of electricity shows its weak side.

Nothing is more emblematic of an electron than its negative charge. It's the trait that determines how the particle behaves in electromagnetic environments, such as a wire's electric field and Earth's magnetic field.

Yet physicists have long known that electrons also respond to a nuclear force, called the weak force, that's responsible for radioactive decay. Now, researchers have directly measured that weak force between pairs of electrons. In so doing, they've determined another kind of charge on the electron, its weak charge.

The fact that electrons, which aren't nuclear particles, respond to a nuclear force at all stems from a deep connection between the electromagnetic and weak forces (SN: 10/16/99, p. 247).

The new results come from experiments at the Stanford (Calif.) Linear Accelerator Center (SLAC). They show that the weak force between electrons is less than a millionth the strength of the electromagnetic force, says Krishna S. Kumar of the University of Massachusetts at Amherst, one of the study's leaders. The measurements were made at an electron-electron separation equivalent to about the width of a proton, he adds.

In an upcoming Physical Review Letters, Kumar and his colleagues report that they repeatedly fired pulses of 500 billion electrons into the end of an aluminum tube filled with liquid hydrogen. Their goal was to knock electrons from those hydrogen atoms. As the scientists generated pulses of electrons, they alternately set each pulse's spin, a quantum trait, to either clockwise or counterclockwise. Following each pulse, detectors on the far side of the tube typically recorded about 10 million electrons unhinged from hydrogen atoms.

The consequences of an impact of an incoming electron on a target electron depend on the strength of electromagnetic repulsion as well as on the energy of the incoming electron, Kumar explains. However, because all electrons have the same tiny weak-force charge, there was a minuscule addition to the repulsion between electrons that contributed to knocking the particles loose from the hydrogen atoms.

Theorists had predicted that the weak force, but not the electromagnetic force, is stronger for electrons with one spin orientation than for those with the other, a phenomenon known as parity violation (SN: 1/15/00, p. 39). In the new study, that disparity manifested itself as a single extra electron emerging from a pulse of electrons with counterclockwise spin compared to a pulse of electrons with clockwise spin. That tiny difference revealed the strength of the electron's weak charge, Kumar notes.

In detecting such a small effect, the team achieved "incredible sensitivity," comments SLAC'S Charles Y. Prescott, a pioneer of similar parity-violation experiments but not a participant in the new experiment.

The SLAC study isn't the first to gauge the electron's weak charge. However, among the few comparable investigations to date, it's the only one that has collided electrons only with other electrons, making its findings relatively straightforward to interpret, Kumar says.

The SLAC experiment is "ushering in a new era of ... very precise measurements," comments Michael J. Ramsey-Musolf of the California Institute of Technology in Pasadena and the University of Connecticut in Storrs. He notes that the improvements that the experimenters devised for SLAC's electron accelerator should also work at other laboratories.