Parity Violation in Electron-Electron Scattering

Parity violation in electron-electron scattering has been seen for the first time, adding to physicists' understanding of the elusive weak force. Parity is name for the proposition that if one viewed an interaction among particles in a special mirror that reflected in all three dimensions then physics would be the same in the ordinary and in the mirror world. Three of the four known physical forces---gravity, electromagnetic, and strong---respect (or "conserve") parity. The fourth force, the weak force, does not conserve parity, a fact established in the 1950s by watching the decays of cobalt nuclei. Since then parity violation has also been observed in other reactions, such as transitions between energy levels within atoms and electron-positron annihilations, but never before in low-angle, relatively low-energy electron-electron scattering.

Electrons are non-nuclear particles; so why do they scatter via any kind of nuclear force, much less the weak nuclear force? Because the weak and electromagnetic forces, though normally very different in their attributes (the electromagnetic force keeps atoms together and governs light, while the weak force exerts itself only at very short range, within about the size of a proton, and is responsible for some kinds of radioactivity) the two forces are still, properly speaking, parts of a single underlying "electroweak" force. Therefore even though electrons interact chiefly through the electromagnetic force, there is enough admixture of weak-force to make itself felt, albeit only in an experiment of great delicacy.

Researchers at SLAC scattered a high-energy beam of polarized electrons off electrons in a liquid hydrogen target and measured the fractional difference in scattering rates when the intrinsic spin of the beam electrons were lined up with or against the direction of the beam. The observed asymmetry not only demonstrated that a bit of parity-violating force was present (in keeping with theoretical ideas about the weak force) but also provided a measure---in fact, the first quantitative measure---of the electrons' "weak charge," a commodity, analogous to electric charge, and indicative of the strength of the weak interaction between two electrons.

One of the team members, Krishna Kumar of the University of Massachusetts (kkumar@physics.umass.edu), asserts that the statistical error of 30 parts per billion (ppb) is the most precise measurement of an asymmetry (the measured effect was 175 parts per billion) in a lepton scattering experiment (that is, one involving electrons, muons, or neutrinos). (Anthony et al., Physical Review Letters, upcoming article.)