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Measurement of the Weak Mixing Angle in Møller Scattering


Bega, Klejda Adnan (2004) Measurement of the Weak Mixing Angle in Møller Scattering. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HVF5-6787.


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 = (σRL)/(σRL), 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.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:electron-electron left-right scattering asymmetry; electroweak theory; fixed target; parity non-conservation; SLAC E158
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hughes, Emlyn Willard (advisor)
  • Kolomensky, Yury (co-advisor)
Thesis Committee:
  • Hughes, Emlyn Willard (chair)
  • Peck, Charles W.
  • Pine, Jerome
  • Frautschi, Steven C.
Defense Date:12 April 2004
Record Number:CaltechETD:etd-04292004-175942
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1543
Deposited By: Imported from ETD-db
Deposited On:13 May 2004
Last Modified:20 Jan 2021 22:41

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