Free Ebook Measurement of the weak mixing angle in Moller scattering

The weak mixing parameter sin2?w 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?w is expected to change with the interaction energy. As a result, a measurement of sin2?w 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?w at low energy is from measuring the left-right, parity-violating asymmetry in electron-electron (Møller) scattering: APV = (?R-?L)/(?R+?L), where ?R and ?L 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 Møller scattering, gee. At tree level gee= 1/4 - sin2?w. A precision measurement of the parity-violating asymmetry in Møller 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 dissertation 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 Møller scattering. This value corresponds to a weak mixing angle at Q2=0.026 (GeV/c)2 of sin2?w MS = 0.2379 ? 0.0016 (stat.) ? 0.0013 (syst.), which is -0.3 standard deviations away from the Standard Model prediction: sin2?wpredictedMS = 0.2385 ? 0.0006 (theory). The E158 measurement of sin2?w at a precision of ?(sin2?w) = 0.0020 provides new physics sensitivity at the TeV scale.

Donloawd Free Ebook Measurement of the weak mixing angle in Moller scattering