Type of Document Dissertation Author Leacock, John Deane Author's Email Address email@example.com URN etd-10172012-113252 Title Measuring the Weak Charge of the Proton and the Hadronic Parity Violation of the N → ∆ Transition Degree PhD Department Physics Advisory Committee
Advisor Name Title Pitt, Mark L. Committee Chair Huber, Patrick Committee Member Khodaparast, Giti A. Committee Member Link, Jonathan M. Committee Member Keywords
- weak charge of the proton
- Standard Model test
- parity violation
Date of Defense 2012-10-16 Availability restricted AbstractQweak 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 θW , 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 → ∆ 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, d∆ .
The elastic asymmetry at Q2 = 0.0252±0.0007 GeV2 was measured to be Aep = −265±40±
22 ± 68 ppb (stat., sys., and blinding). Extrapolated to Q2 = 0, the value of the proton’s
weak charge was measured to be QpW = 0.077 ± 0.019 (stat. and sys.)±0.026 (blinding). This
is within 1 σ of the Standard Model prediction of QpW = 0.0705 ± 0.0008.
The N → ∆ inelastic asymmetry at Q2 = 0.02078 ± 0.0005 GeV2 and W = 1205 MeV was
measured to be Ainel = −3.03 ± 0.65 ± 0.73 ± 0.07 ppm (stat., sys., and blinding). This
result constrains the low energy constant to be d∆ = 5.8 ± 22gπ , and, if the result of the G0
experiment is included, d∆ = 5.8 ± 17gπ . This result rules out suggested large values of d∆
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 ∆ resonance. It is currently the best constraint for the low energy constant,
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