Spin-polarized beams of high-energy electrons scattering off unpolarized electrons or nuclei are particularly useful for examining the very slight preference for left handers in the realm of fundamental particles. But because that preference is measured in parts per 10 million, such experiments require extraordinary care. For longitudinally polarized electron spins, any difference between the scattering of electrons spinning like left- and right-handed screws violates parity conservation—that is, mirror symmetry. Electromagnetic interactions, which dominate electron scattering, strictly respect parity conservation. But the weak interactions do not.

In 2002, the E158 collaboration at SLAC began measuring the tiny fractional difference between the cross sections for the elastic scattering of left- and right-polarized 50-GeV electrons off electrons in an unpolarized liquid-hydrogen target. The polarized beam electrons were accelerated in the laboratory’s 3-km-long linac. The collaboration’s final report, just published, gives a right–left asymmetry measurement precise enough to demonstrate for the first time that...

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