Ross Young

JLab, Newport News

Triumph of the Standard Model to All-New Precision in Parity-Violating Electron Scattering

The Standard Model has proven an extremely successful theory in describing the behaviour of nuclear and particle physics. The theoretical desire to restore simplicity in the fundamental theory of nature has continued to motivate the experimental search for new physics. Within the strong interaction, described by quantum chromodynamics (QCD), the experimental knowledge of the low-energy sector is typically far superior to what can be determined directly from theory -- which limits the ability to perform precision tests. One aspect of low-energy QCD, namely the strange-quark content of the nucleon, has presented a significant challenge to both theory and experiment alike for many years. The latest advances in the analysis of lattice QCD results and precision parity-violation measurements have seen theory and experiment make side-by-side improvement in our knowledge of the strangeness electromagnetic form factors of the nucleon. Unlike the strong interaction, the electroweak sector is very well described theoretically, and it is precisely this knowledge that has enabled the isolation of strangeness in polarised electron scattering measurements. With such a tightly constrained theory in this sector, it opens the opportunity for precision low-energy measurements to reveal (or constrain) new physics beyond the Standard Model. While this is the goal of the future Qweak and Moeller measurements at Jefferson Lab, we demonstrate that the precision and kinematic coverage offered by current strangeness measurements already provide a significant improvement in the experimental knowledge of the weak interaction at low energies. In combination with existing measurements, primarily from atomic parity violation, the implications of these results limit the mass scale of new physics to beyond ~1-5 TeV.

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