Title page for ETD etd-10172012-113252

Type of Document Dissertation
Author Leacock, John Deane
Author's Email Address leacock@vt.edu
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
  • weak charge of the proton
  • Standard Model test
  • Qweak
  • parity violation
Date of Defense 2012-10-16
Availability restricted
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 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,

d∆ .

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