Mirror, mirror. Thanks to a quirk of the weak nuclear force, the laws of particle physics aren't mirror-symmetric. Now, physicists have shown the effect using only electrons.

Look Ma, No Nuclei

Without using atomic nuclei, physicists have demonstrated a strange asymmetry in the weak nuclear force, a fundamental force of nature. The trick neatly confirms the reigning theory of particle physics--which may disappoint many physicists.

In everyday experience, any physical interaction is just as possible as its mirror reflection, so that for every watch that ticks clockwise, an inspired watchmaker can build an exact mirror image that runs counterclockwise. But that commonsense notion doesn't necessarily apply to subatomic particles. In a certain type of nuclear decay, an atomic nucleus spits out an electron and a particle called an antineutrino that always spins to the right as it zings along, like a football flung by a right-handed quarterback. The mirror-image decay, in which the antineutrino spins to the left, never occurs. That asymmetry is known as parity violation and is a defining characteristic of the weak nuclear force, which causes such radioactive decay.

The asymmetry should also show up in the interactions between electrons. According to the Standard Model of particle physics, the electromagnetic force with which electrons push each other and the weak nuclear force are really two aspects of a single underlying "electroweak" force that bleed into each other. So every once in a while, two electrons should push on each other through the parity-violating weak force.

And that's just what happens, reports a team of 59 physicists working at the Stanford Linear Accelerator Center (SLAC) in Menlo Park, California. The team shot a beam of electrons spinning one way or the other into a chamber filled with hydrogen gas and measured the rate at which the incoming electrons bounced off the electrons in the hydrogen atoms. The researchers looked for subtle differences in the probability that right-spinning and left-spinning electrons would ricochet slightly. The Standard Model predicts a slight bias in these probabilities, and after a mere 90 million trials, the team found it: left-spinning electrons were 0.0000175% more likely to richochet than right-spinning ones, just as predicted. The teams report the results in a paper to be published in Physical Review Letters.

"It's a really nice confirmation of the Standard Model," says William Marciano, a theoretical physicist at Brookhaven National Laboratory in Upton, New York. And that's not such great news for physicists hoping for a discrepancy that might point to new particles, says Krishna Kumar of the University of Massachusetts, Amherst, spokesperson for the experimental team. "There's no question that I was disappointed."

Related Sites
The E158 home page at SLAC