Measuring the Weak Charge of the Proton and the Hadronic Parity Violation of the N â â Transition
Leacock, John Deane
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Qweak will determine the weak charge of the proton, QpW, via an asymmetry measurement of parity-violating elastic electron-proton scattering at low four momentum transfer to a precision of 4%. QpW has a firm Standard Model prediction and is related to the weak mixing angle, sin2 Î¸W , a well-defined Standard Model parameter. Qweak will probe a subset of new physics to the TeV mass scale and test the Standard Model. The details of how this measurement was performed and the analysis of the 25% elastic dataset will be presented in this thesis. Also, an analysis of an auxiliary measurement of the parity-violating asymmetry in the N â â transition is presented. It is used as a systematic inelastic background correction in the elastic analysis and to extract information about the hadronic parity violation through the low energy constant, dâ . The elastic asymmetry at Q2 = 0.0252Â±0.0007 GeV2 was measured to be Aep = â 265Â±40Â± 22 Â± 68 ppb (stat., sys., and blinding). Extrapolated to Q2 = 0, the value of the protonâ s weak charge was measured to be QpW = 0.077 Â± 0.019 (stat. and sys.)Â±0.026 (blinding). This is within 1 Ï of the Standard Model prediction of QpW = 0.0705 Â± 0.0008. The N â â inelastic asymmetry at Q2 = 0.02078 Â± 0.0005 GeV2 and W = 1205 MeV was measured to be Ainel = â 3.03 Â± 0.65 Â± 0.73 Â± 0.07 ppm (stat., sys., and blinding). This result constrains the low energy constant to be dâ = 5.8 Â± 22gÏ , and, if the result of the G0 experiment is included, dâ = 5.8 Â± 17gÏ . This result rules out suggested large values of dâ motivated by radiative hyperon decays. The elastic measurement is the first direct measurement of the weak charge of the proton while the inelastic measurement is only the second measurement of the neutral current excitation of the â resonance. It is currently the best constraint for the low energy constant, dâ .
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