Measurement of the Weak Mixing Angle in Møller Scattering

Citation

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. https://resolver.caltech.edu/CaltechETD:etd-04292004-175942

Abstract

The weak mixing parameter sin²θ_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 sin²θ_w is expected to change with the interaction energy. As a result, a measurement of sin²θ_w at low energy (Q² << m_Z, where Q² is the momentum transfer and m_Z 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 sin²θ_w at low energy is from measuring the left-right, parity-violating asymmetry in electron-electron (Møller) scattering: A_PV = (σ_R-σ_L)/(σ_R+σ_L), where σ_R and σ_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, g_ee. At tree level g_ee= 1/4 - sin²θ_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 A_PV 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 A_PV = -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 sin²θ_w [...] = 0.2379 ± 0.0016 (stat.) ± 0.0013 (syst.), which is -0.3 standard deviations away from the Standard Model prediction: sin²θ_w^predicted[...] = 0.2385 ± 0.0006 (theory). The E158 measurement of sin²θ_w at a precision of δ(sin²θ_w) = 0.0020 provides new physics sensitivity at the TeV scale.

Thesis Files