Southpaw electrons just have to be different from the righties

BY HEATHER ROCK WOODS

Just as left-handed people are statistically more likely to have accidents, "left-handed" electrons behave slightly differently than their "right-handed" counterparts. They are 10 percent more likely to exchange a Z particle (a carrier of the weak force) with another electron.

An extraordinarily precise and challenging experiment at the Stanford Linear Accelerator Center (SLAC) that ended last month has proved for the first time that this asymmetry exists in electron-electron interactions -- validating one prediction in physics' theoretical Standard Model.

"Generating the experiment's first precision measurement was like searching 10 million haystacks to pinpoint the single one that contains the needle," said staff physicist Mike Woods, who coordinated the experiment's operations.

The amount of asymmetry found contributes to a measurement called the electroweak mixing angle, which describes the strength of the weak force. The weak force, transmitted by Z and W particles, is responsible for some types of radioactive decay. The mixing angle's expected value is 0.238. The experiment, called E-158, released preliminary results this spring from its first set of data. The final results, reflecting 5 months of data taken over 18 months, ultimately will measure the mixing angle with a relative error of 0.5 percent.

"We're hoping to see something that doesn't agree with the Standard Model prediction," said experiment spokesman Krishna Kumar, associate professor of physics at the University of Massachusetts-Amherst. "If there is new physics, we need to get the error bar down to tell," which is exactly what E-158's analysis team, headed by Yury Kolomensky, assistant professor of physics at the University of California-Berkeley, is fervently working on.

The experiment is a collaboration of about 65 scientists from 11 institutions.

Pulses of 500 billion electrons sped down the 2-mile-long linear accelerator and bombarded a target of liquid hydrogen at the end of the accelerator every 8 milliseconds. In each pulse, the electrons were polarized to be either right- or left-handed.

Right- and left-handed electrons have opposite angular momentum; they are the mirror image of each other the way our hands are. Until the 1950s, physicists assumed that the weak force had mirror symmetry -- that the mirror world behaved the same as the real world. For example, the mirror image of a top spinning clockwise (which appears counter-clockwise in the mirror) would act the same as a top spinning counter-clockwise.

In 1977, SLAC Professor Charles Prescott did the first experiment that found this was not true for Z particles in electron-quark interactions. The weak force is the only force that violates mirror symmetry (called parity). The electromagnetic and strong forces conserve parity, and gravity is believed to do so as well.

E-158 is the first experiment to test the asymmetry with electron-electron scattering. Most of the electrons zoomed through the target and touched nothing. Some electrons scattered (or deflected) electrons at rest in the target by exchanging photons. A very small proportion of incoming electrons scattered by exchanging a Z particle. In scattering, electrons don't collide; they bend away from each other the way two cars merging into the same spot veer away from each other to avoid a crash.

The experiment compensated for the small asymmetry (one Z exchange in 10 million scattering events) by generating a high rate of scattering events. A left-handed pulse of 500 billion electrons produced about 11 Z exchanges, compared to a right-handed pulse producing about 10.

"In order to measure this number accurately, we need to repeat the comparison of left- and right-handed pulses 400 million times," said Kumar.

When Kumar first proposed looking for incredibly tiny effects at the lower energies of a fixed target experiment, many physicists were skeptical that even SLAC's advanced apparatus could produce the necessary experimental conditions. But SLAC physicists and engineers were able to meet the exacting requirements. They generated one crucial component -- a high-energy, narrow beam of polarized electrons. That accomplishment bodes well for the difficult beam needed in the Next Linear Collider, the world physics community's next big project.