A Measurement of the Weak Charge of the Proton through Parity Violating Electron Scattering using the Qweak Apparatus

Abstract

After a decade of preparations, the Qweak experiment at Jefferson Lab is making the first direct measurement of the weak charge of the proton, Qweak9. Because this quantity is suppressed in the Standard Model, a 4% result will significantly constrain new physics at the TeV scale while providing the most precise measurement of sin2θW at low energies. Operationally, we measure the small (about −0.220ppm) parity violating asymmetry in electron–proton scattering in integrating mode while flipping the longitudinal polarization of the electrons up to 1000 times per second. Potential sources of new, parity violating interactions between electrons and light quarks include a Z′, lepto-quarks, and parity violating SUSY interactions. The result presented here is based on the data taken during an initial two weeks period which included a 16.7% measurement of the parity violating electron–proton (e→p) scattering asymmetry, A=−0.279±0.035(stat.)±0.031(syst.)ppm at Q2=0.0250±0.0006(GeV)2. The weak charge of the proton is extracted by performing a global analysis on parity violating electron scattering (PVES) asymmetries on nuclear targets and it is QWp=0.064±0.012. Then effective vector couplings of the up/down quarks (C1u/C1d) and weak charge of the neutron are extracted by combining precise 133Cs atomic parity violating (APV) measurement and PVES measurements. The result is a proof of principle for the analysis of the full Qweak data to be completed in the near future.