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Boffins testing the strength of the 'weak' charge of electrons have seen parity violations in electron-electron scattering for the first time. The results could eventually lead to a better understanding of why things have mass.

In very simple terms, parity is a measure of the left-right (or mirror) symmetry of nature. Its origins lie with Louis Pasteur's discovery of optical isomers: the ability of left or right handed molecules to rotate polarised light in opposite directions. Generally, left-handed and right-handed versions of things exist in equal quantities.

In fact, parity is conserved in three of the four fundamental forces: gravity, electromagnetism and the strong force are all symmetric. But the weak force is not - a surprising result when first observed in a famous 1956 experiment with the radioactive decay of cobalt.

The scientists used the Stanford Linear Accelerator (SLAC) to fire polarised electron beams at a liquid hydrogen target. They measured the rate at which the polarised electrons bounced off target electrons for both left and right handed polarisations. With this data, they could look for asymmetry in the result: i.e. did the target scatter left or right handed polarised electrons more?

They observed a difference of 175 parts per billion: a very slight tendency to scatter more of one than the other. Using this data, they calculated that the strength of the so-called weak charge was -0.053, plus or minus 0.011. The standard model predicts a strength of -0.046, so this is very good agreement.

Similar results have been obtained by smashing polarized electrons into positrons and creating Z0 particles (neutral particles that transmit the weak force), but where the difference is very large ~10%. What's new about this experiment is that is able to observe such a tiny asymmetry to yield a precise measurement of the weak charge at a very different energy scale, much below what is needed to directly create the Z0 particles.

Mike Woods, a scientist working on the experiment at SLAC, explained that this is important because precise measurements at different energy scales are needed to look for certain new physics effects. He noted: "Weak charge measurements are very topical in particle physics, in part because they give the best indirect constraints on the Higgs mass".

The Higgs boson, if it exists, would help explain why matter has mass. It is as yet only theoretical. If scientists can work out how heavy it is, they will know where to go looking for it. ®

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