Phd records
https://feeds.library.caltech.edu/people/Bega-Klejda-Adnan/Phd.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenWed, 31 Jan 2024 18:49:55 +0000Measurement of the Weak Mixing Angle in Møller Scattering
https://resolver.caltech.edu/CaltechETD:etd-04292004-175942
Authors: {'items': [{'id': 'Bega-Klejda-Adnan', 'name': {'family': 'Bega', 'given': 'Klejda Adnan'}, 'show_email': 'NO'}]}
Year: 2004
DOI: 10.7907/HVF5-6787
<p>The weak mixing parameter sin<sup>2</sup>θ<sub>w</sub> is one of the fundamental parameters of the Standard Model. Its tree-level value has been measured with high precision at energies near the Z<sup>0</sup> pole; however, due to radiative corrections at the one-loop level, the value of sin<sup>2</sup>θ<sub>w</sub> is expected to change with the interaction energy. As a result, a measurement of sin<sup>2</sup>θ<sub>w</sub> at low energy (Q<sup>2</sup> << m<sub>Z</sub>, where Q<sup>2</sup> is the momentum transfer and m<sub>Z</sub> 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.</p>
<p>One way of obtaining sin<sup>2</sup>θ<sub>w</sub> at low energy is from measuring the left-right, parity-violating asymmetry in electron-electron (Møller) scattering: A<sub>PV</sub> = (σ<sub>R</sub>-σ<sub>L</sub>)/(σ<sub>R</sub>+σ<sub>L</sub>), where σ<sub>R</sub> and σ<sub>L</sub> 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, g<sub>ee</sub>. At tree level g<sub>ee</sub>= 1/4 - sin<sup>2</sup>θ<sub>w</sub>. 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 Q<sup>2</sup> ~ 0.03 (GeV/c)<sup>2</sup>. The tree-level prediction for A<sub>PV</sub> at such energy is =~ 300 ppb. However, one-loop radiative corrections reduce its value by ~40%.</p>
<p>This dissertation reports the E158 results from the 2002 data collection period. The parity-violating asymmetry was found to be A<sub>PV</sub> = -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 Q<sup>2</sup>=0.026 (GeV/c)<sup>2</sup> of sin<sup>2</sup>θ<sub>w</sub> [...] = 0.2379 ± 0.0016 (stat.) ± 0.0013 (syst.), which is -0.3 standard deviations away from the Standard Model prediction: sin<sup>2</sup>θ<sub>w</sub><sup>predicted</sup>[...] = 0.2385 ± 0.0006 (theory). The E158 measurement of sin<sup>2</sup>θ<sub>w</sub> at a precision of δ(sin<sup>2</sup>θ<sub>w</sub>) = 0.0020 provides new physics sensitivity at the TeV scale.</p>
https://thesis.library.caltech.edu/id/eprint/1543