Measurement of the Weak Mixing Angle in Moller Scattering

The weak mixing parameter, sin2 θw, is one of the fundamental parameters of the Standard Model. Its tree-level value has been measured with high precision at energies near the Z0 pole; however, due to radiative corrections at the one-loop level, the value of sin2 θw is expected to change with the interaction energy. As a result, a measurement of sin2 θw at low energy (Q2 << mZ, where Q2 is the momentum transfer and mZ is the Z boson mass), provides a test of the Standard Model at the one-loop level, and a probe for new physics beyond the Standard Model. One way of obtaining sin2 θw at low energy is from measuring the left-right, parity-violating asymmetry in electron-electron (Moeller) scattering: APV = σR-σL/σR+σL, where σR and σL are the cross sections for right- and left-handed incident electrons, respectively. The parity violating asymmetry is proportional to the pseudo-scalar weak neutral current coupling in Moeller scattering, gee. At tree level gee = (1/4 -sin2 θw). A precision measurement of the parity-violating asymmetry in Moeller scattering was performed by Experiment E158 at the Stanford Linear Accelerator Center (SLAC). During the experiment, ∼50 GeV longitudinally polarized electrons scattered off unpolarized atomic electrons in a liquid hydrogen target, corresponding to an average momentum transfer Q2 ∼ 0.03 (GeV/c)2. The tree-level prediction for APV at such energy is ∼300 ppb. However one-loop radiative corrections reduce its value by ∼40%. This document reports the E158 results from the 2002 data collection period. The parity-violating asymmetry was found to be APV = -160 ± 21 (stat.) ± 17 (syst.) ppb, which represents the first observation of a parity-violating asymmetry in Moeller scattering. This value corresponds to a weak mixing angle at Q2 = 0.026 (GeV/c)2 of sin2 θwovrMS = 0.2379 ± 0.0016 (stat.) ± 0.0013 (syst.), which is -0.3 standard deviations away from the Standard Model prediction: sin2 θwovrMSpredicted = 0.2385 ± 0.0006 (theory). The E158 measurement of sin2 θw at a precision of (delta)(sin2 θw) = 0.0020 provides new physics sensitivity at the TeV scale