Radiative corrections and parity-violating electron-nucleon scattering

Radiative corrections to the parity-violating asymmetry measured in elastic electron-proton scattering are analyzed in the framework of the Standard Model. We include the complete set of one-loop contributions to one quark current amplitudes. The contribution of soft photon emission to the asymmetry is also calculated, giving final results free of infrared divergences. The one quark radiative corrections, when combined with previous work on many quark effects and recent SAMPLE experimental data, are used to place some new constraints on electroweak form factors of the nucleon

Contribution of spin 1/2 and 3/2 resonances to two-photon exchange effects in elastic electron-proton scattering

We calculate contributions of hadron resonances to two-photon exchange effects in electron-proton scattering. In addition to the nucleon and P33 resonance, the following heavier resonances are included as intermediate states in the two-photon exchange diagrams: D13, D33, P11, S11 and S31. We show that the corrections due to the heavier resonances are smaller that the dominant nucleon and P33 contributions. We also find that there is a partial cancellation between the contributions from the spin 1/2 and spin 3/2 resonances, which results in a further suppression of their aggregate two-photon exchange effect.Comment: 6 pages, 1 figure; additional comparison with data, results unchanged; to be published in Phys. Rev.

Δ\Delta resonance contribution to two-photon exchange in electron-proton scattering

We calculate the effects on the elastic electron-proton scattering cross section of the two-photon exchange contribution with an intermediate $\Delta$ resonance. The $\Delta$ two-photon exchange contribution is found to be smaller in magnitude than the previously evaluated nucleon contribution, with an opposite sign at backward scattering angles. The sum of the nucleon and $\Delta$ two-photon exchange corrections has an angular dependence compatible with both the polarisation transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors.Comment: 9 pages, 3 figures, RevTeX4; more complete discussion of results, conclusions unchanged; to be published in Physical Review Letter

Two-photon exchange and elastic electron-proton scattering

Two-photon exchange contributions to elastic electron-proton scattering cross sections are evaluated in a simple hadronic model including the finite size of the proton. The corrections are found to be small in magnitude, but with a strong angular dependence at fixed $Q^2$. This is significant for the Rosenbluth technique for determining the ratio of the electric and magnetic form factors of the proton at high $Q^2$, and partly reconciles the apparent discrepancy with the results of the polarization transfer technique.Comment: 4 pages, 4 figures, v2: additional references and minor clarifications in text, accepted in Physical Review Letter

A novel approach to the global analysis of proton form factors in elastic electron-proton scattering

We present a novel method for the global analysis of elastic electron-proton scattering when combining data sets from experiments with different overall normalization uncertainties. The method is a modification of one employed by the NNPDF collaboration in the fitting of parton distribution data. This method is an alternative to the 'penalty trick' method traditionally employed in global fits to proton electric and magnetic form factors, while avoiding the biases inherent in that approach. We discuss issues that arise when extending the method to nonlinear models. For data with $Q^2>1$ GeV$^2$ we find relatively minor differences to traditional model fits when the normalization uncertainties from different experiments are correctly accounted for. We discuss implications of this method for the well-known discrepancy between the form factor ratio $G_E/G_M$ extracted from the Rosenbluth and polarization transfer techniques.Comment: 5 pages, 4 figures, 2 table