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

摘要

After almost five decades of study of parity violation in hadronic systems,the determination of the seven weak meson-nucleon couplings is stillincomplete. Whereas parity violation in nuclear systems is complicated by theintricacies of QCD, measurements of parity violation in the much simplerproton-proton system are more straightforward to interpret. We now have threesuch precision pp experiments at 13.6, at 45, and 221 MeV. Today there are alsobetter possibilities for theoretical interpretation using effective fieldtheory. In electron-proton scattering, parity violating ep experiments such asSAMPLE, G0, HAPPEX, and PVA4 have already shown that the strange quarkcontributions to the charge and magnetization distributions of the nucleon aretiny. When analyzed together, the results have also greatly improved knowledgeof the proton's "weak charge" (Q^p_weak = 1-4sin^2\theta_W at tree level). TheQ^p_weak experiment at JLab will further improve this, determining the proton'sweak charge to a precision of about 4%. Such a precision will either establishconformity with the Standard Model of quarks and leptons or point to NewPhysics. Following the upgrade of CEBAF at JLab to 12 GeV, a parity violatingelectron-electron (Moller) scattering experiment similar to SLAC E158, willmeasure the weak charge of the electron and hence sin^2\theta_W at low energywith a precision comparable to the most precise individual measurements at theZ0 pole (to about +/- 0.00025). This experiment will be complementary toQ^p_weak in terms of sensitivity to New Physics.