Measurement of the Weak Mixing Angle in Møller Scattering

Author: Bega, Klejda Adnan

Year: 2004

Degree: Dissertation (Ph.D.)

Advisors: Hughes, Emlyn Willard; Kolomensky, Yury

Committee Members: Hughes, Emlyn Willard; Peck, Charles W.; Pine, Jerome; Frautschi, Steven C.

Option: Physics

DOI: 10.7907/HVF5-6787

Abstract

The weak mixing parameter sin2θw 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 sin2θw is expected to change with the interaction energy. As a result, a measurement of sin2θw 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 sin2θw at low energy is from measuring the left-right, parity-violating asymmetry in electron-electron (Møller) 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 Møller scattering, gee. At tree level gee= 1/4 - sin2θw. 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 sin2θw [...] = 0.2379 ± 0.0016 (stat.) ± 0.0013 (syst.), which is -0.3 standard deviations away from the Standard Model prediction: sin2θwpredicted[...] = 0.2385 ± 0.0006 (theory). The E158 measurement of sin2θw at a precision of δ(sin2θw) = 0.0020 provides new physics sensitivity at the TeV scale.

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