Measurement of parity violation in electron–quark scattering

A high-precision parity-violating electron–quark scattering experiment provides measurements of a combination of electron–quark weak couplings with a precision five times higher than the single previous direct study, confirming the predictions of the electroweak particle-physics theory and providing constraints on parity-violating interactions beyond the standard model. Parity-violating asymmetry revisited Parity symmetry — or mirror-image symmetry — implies that flipping left and right does not change the laws of physics. Violation of parity symmetry in the weak nuclear force was discovered in the mid-1950s and parity violation in electron scattering was important in establishing, and is now used to test, the standard model of particle physics. This study reports a high-precision electron–quark scattering experiment that provides a measurement of the parity-violating asymmetry with a precision of five times higher than the single previous direct study via this scattering process. The results confirm the predictions of electroweak particle-physics theory, while providing constraints on parity-violating interactions beyond the standard model. Symmetry permeates nature and is fundamental to all laws of physics. One example is parity (mirror) symmetry, which implies that flipping left and right does not change the laws of physics. Laws for electromagnetism, gravity and the subatomic strong force respect parity symmetry, but the subatomic weak force does not 1 , 2 . Historically, parity violation in electron scattering has been important in establishing (and now testing) the standard model of particle physics. One particular set of quantities accessible through measurements of parity-violating electron scattering are the effective weak couplings C 2 q , sensitive to the quarks’ chirality preference when participating in the weak force, which have been measured directly 3 , 4 only once in the past 40 years. Here we report a measurement of the parity-violating asymmetry in electron–quark scattering, which yields a determination of 2 C 2 u − C 2 d (where u and d denote up and down quarks, respectively) with a precision increased by a factor of five relative to the earlier result. These results provide evidence with greater than 95 per cent confidence that the C 2 q couplings are non-zero, as predicted by the electroweak theory. They lead to constraints on new parity-violating interactions beyond the standard model, particularly those due to quark chirality. Whe