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Published July 2011 | Published
Journal Article Open

Model dependence of the γ Z dispersion correction to the parity-violating asymmetry in elastic ep scattering


We analyze the dispersion correction to elastic parity violating electron-proton scattering due to γZ exchange. In particular, we explore the theoretical uncertainties associated with modeling contributions of hadronic intermediate states. Taking into account constraints from low- and high-energy, parity-conserving electroproduction measurements, choosing different models for contributions from the nonresonant processes, and performing the corresponding flavor rotations to obtain the electroweak amplitude, we arrive at an estimate of the uncertainty in the total contribution to the parity-violating asymmetry. At the kinematics of the Q-Weak experiment, we obtain a correction to the asymmetry equivalent to a shift in the proton weak charge of (0.0054±0.0020). This should be compared to the value of the proton's weak charge of QW^p=0.0713±0.0008 that includes Standard Model contributions at tree level and one-loop radiative corrections. Therefore, we obtain a new Standard Model prediction for the parity-violating asymmetry in the kinematics of the Q-Weak experiment of (0.0767±0.0008±0.0020_(γZ)). The latter error leads to a relative uncertainty of 2.8% in the determination of the proton's weak charge and is dominated by the uncertainty in the isospin structure of the inclusive cross section. We argue that future parity-violating inelastic ep asymmetry measurements at low to moderate Q^2 and W^2 could be exploited to reduce the uncertainty associated with the dispersion correction. Because the corresponding shift and error bar decrease monotonically with decreasing beam energy, a determination of the proton's weak charge with a lower-energy experiment or measurements of "isotope ratios" in atomic parity violation could provide a useful cross-check on any implications for physics beyond the Standard Model derived from the Q-Weak measurement.

Additional Information

© 2011 American Physical Society. Received 1 March 2011; revised 1 June 2011; published 29 July 2011. Authors are grateful to W. Melnitchouk and C. Carlson for useful discussions and comments. This work was supported in part by US DOE Contract No. DE-FG02-87ER40365 and NSF Grant No. Phy-0854805 (C.J.H. and M.G.) and by US DOE Contract No.DE-FG02-08ER41531(M.J.R-M).M.J.R.M. also gratefully acknowledges support from the Wisconsin Alumni Research Foundation and the Aspen Center for Physics, where part of this work was carried out.

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