From Hadronic Parity Violation to Parity-Violating Electron Scattering and Tests of the Standard Model

TL;DR

This paper reviews the progress in measuring parity violation in hadronic and leptonic systems, highlighting recent experiments and their implications for testing the Standard Model and probing for new physics.

Contribution

It summarizes recent experimental results and discusses future prospects for precision measurements of weak charges in protons and electrons using parity-violating scattering experiments.

Findings

Precision pp experiments at 13.6, 45, and 221 MeV provide insights into weak meson-nucleon couplings.

Electron-proton scattering experiments show tiny strange quark contributions to nucleon structure.

Upcoming experiments aim to measure weak charges with high precision to test the Standard Model and search for new physics.

Abstract

After almost five decades of study of parity violation in hadronic systems, the determination of the seven weak meson-nucleon couplings is still incomplete. Whereas parity violation in nuclear systems is complicated by the intricacies of QCD, measurements of parity violation in the much simpler proton-proton system are more straightforward to interpret. We now have three such precision pp experiments at 13.6, at 45, and 221 MeV. Today there are also better possibilities for theoretical interpretation using effective field theory. In electron-proton scattering, parity violating ep experiments such as SAMPLE, G0, HAPPEX, and PVA4 have already shown that the strange quark contributions to the charge and magnetization distributions of the nucleon are tiny. When analyzed together, the results have also greatly improved knowledge of the proton's "weak charge" (Q^p_weak = 1-4sin^2\theta_W at tree level). The Q^p_weak experiment at JLab will further improve this, determining the proton's weak charge to a precision of about 4%. Such a precision will either establish conformity with the Standard Model of quarks and leptons or point to New Physics. Following the upgrade of CEBAF at JLab to 12 GeV, a parity violating electron-electron (Moller) scattering experiment similar to SLAC E158, will measure the weak charge of the electron and hence sin^2\theta_W at low energy with a precision comparable to the most precise individual measurements at the Z0 pole (to about +/- 0.00025). This experiment will be complementary to Q^p_weak in terms of sensitivity to New Physics.