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.

Presented at a SLAC seminar this spring, the first results from the extraordinarily precise and challenging E-158 experiment prove for the first time that this asymmetry exists in electron-electron interactions. Standard Model theory had predicted this outcome. Generating the experiment’s first precision measurement was like searching 10 million haystacks to pinpoint the single one that contains the needle.

The E-158 spectrometer in the final stages of construction before installation of the shielding. The target chamber is on the left, followed by the three dipole magnets (blue) and three of the four quadrupoles (red). (Photo courtesy of EFD)

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. This experiment will ultimately measure it with a relative error of 1/2 percent. SLD made a precise measurement of this value in the last decade at greater energy, where Zs were created, not exchanged. The different conditions mean E-158 is a different test of the Standard Model.

"We’re hoping to see something that doesn’t agree with the Standard Model prediction. All the time, physics measurements are consistent with Standard Model predictions, but we’ve known for 20 years that it’s not completely right," said experiment Spokesman Krishna (KK) Kumar, (University of Massachusetts, Amherst). "If there is new physics, we need to get the error bar down to tell," which is exactly what the E-158’s analysis team, headed by Yury Kolomensky (UC Berkeley), is fervently working on.

E-158 began commissioning at SLAC in 2000 in End Station A (ESA) at the end of the linac. During the third and final physics run this summer, a pulse of 500 billion polarized electrons bombards a target of liquid hydrogen every 8 milliseconds. In each pulse, the electrons are 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 that is spinning clockwise (and thus looks counter-clockwise in the mirror) would act the same as a top spinning counter-clockwise. In 1977, Charles Prescott (EA) did the first experiment (E-122) finding this was not true for Z particles in electron-quark interactions. The weak force is the only force that has parity (mirror) violation. The electromagnetic and strong forces conserve parity, and gravity is believed to do so as well.

"SLAC is a great place to do this experiment because the Lab has a history of studying parity violation with Z particles and produces the highest energy polarized electron beam in the world," Kumar said.

E-158 is the first experiment to test the asymmetry with electron-electron scattering (called Möller scattering). Most of the electrons zoom through the target, touching nothing. Some electrons scatter (or deflect) electrons at rest in the target by exchanging photons. A very small proportion of incoming electrons scatter by exchanging a Z particle. In scattering, the 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 deflection is powered by the electromagnetic force (a photon exchange) or by the weak force (a Z exchange).

The experiment compensates for the small asymmetry (10^-7, or 0.0000001) by generating a high rate of scattering events. For every 500 billion electrons that strike the target, the detector sees 10 million scattered electrons, about 10 of which involve Z exchanges. (The other 9,999,990 are from photon exchanges.) A left-handed pulse will produce 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 he 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.

"SLAC has created a superb low jitter, high-current beam with 85 percent polarization," said Run Coordinator Mike Woods (EA), who worked with Accelerator Department physicists Jim Turner, Franz-Josef Decker and Roger Erickson to implement the exacting beam requirements. Many E-158 beam parameters approach the Next Linear Collider’s requirements for a high current beam, and demonstrate that they are achievable.

To make the measurement possible in the extreme radiation environment created by the beam traveling through the world’s longest liquid hydrogen target, E-158 collaborators built a new type of detector made of copper and fused silica fibers. Similar detectors are being tested for experiments at the Large Hadron Collider being built at CERN. The Experimental Facilities Department (EFD) installed and maintains the E-158 experimental apparatus in ESA, including the target, which is kept at about 18 degrees Kelvin and requires careful handling.

"It really takes the cooperative efforts of many groups throughout the Laboratory to make the experiment a success," said Run Coordinator Michael (MO) Olson (EFD).