Higher-order QED effects play an important role in precision measurements of nucleon elastic form factors in electron scattering. Here we introduce a two-photon exchange QED correction to the parity-violating polarization asymmetry of elastic electron-proton scattering. We calculate this correction in the parton model using the formalism of generalized parton distributions, and demonstrate that it can reach several per cent in certain kinematics, becoming comparable in size with existing experimental measurements of strange-quark effects in the proton neutral weak current.
We compute the corrections from two-photon and \gamma-Z exchange in parity-violating elastic electron--proton scattering, used to extract the strange form factors of the proton. We use a hadronic formalism that successfully reconciled the earlier discrepancy in the proton's electron to magnetic form factor ratio, suitably extended to the weak sector. Implementing realistic electroweak form factors, we find effects of the order 2-3% at Q^2
Two methods, Rosenbluth separation and polarization transfer, can be used to extract the proton form factor ratio [mu]p GEp/GMp, but they do not yield the same results. It is thought that the disagreement is due to two photon exchange corrections to the differential cross sections. High precision proton Rosenbluth extractions were carried out at 102 kinematics points spanning 16 values of momentum transfer Q2, from 0.40 to 5.76 GeV2. Reduced cross sections were found to 1.1% or better for Q2 less than 3 GeV2 increasing to 4% at 5.76 GeV2 The form factor ratios were determined to 1:5-3% for Q2
Elastic electron-proton scattering has long been the tool of choice for the study of the proton form factors, GE(Q 2 ) and GM(Q2 ), which describe the electric and magnetic distributions of the proton as a function of momentum transfer. Recent experiments, measuring the form factors from polarization observables in polarized elastic electron-proton scattering, have found values of the ratio GE(Q2 )/GM(Q2) at high Q2 that contradict the results from unpolarized measurements. A proposed explanation for this discrepancy is the unaccounted two-photon exchange radiative correction, which could affect the unpolarized measurements. As this effect is currently not possible to calculate in a model-independent way, the OLYMPUS experiment was designed to make a direct measurement of it by measuring the elastic positron-proton to electron-proton scattering cross section ratio. The experiment was run in 2012 at DESY using the BLAST spectrometer and the DORIS positron and electron beams at 2 GeV incident on a gaseous hydrogen target. To analyze the data, a careful reconstruction of the scattering events, detailed simulation of the experimental setup, and full radiative corrections to the measured cross sections were performed. Preliminary results for the experiment show a statistically significant two-photon exchange effect, increasing over the measurement range of 0.6 GeV2
Measurements of the electric-to-magnetic ratio of the proton's elastic form factors, GE/GM, have recently been shown to diverge at high momentum transfer, Q2, based on which of two experimental methods is employed. Reanalysis of all results might be able to bring them into agreement if some previously neglected correction were introduced whose magnitude increases with Q2. In the high-energy community, two-photon exchange has become the favored candidate for such a correction. But theoretical models disagree on the size and form of a two-photon exchange correction term entering into elastic lepton-proton scattering and existent empirical data is not sufficiently precise to settle the matter. OLYMPUS is one of several new experiments designed to evaluate R2[gamma], the ratio of the elastic cross sections for positron-proton and electron-proton scattering, whose difference from unity provides a direct measure of the amplitude of the two-photon exchange matrix elements. In this dissertation, OLYMPUS is considered from its motivation, construction, and operation to its analysis, along with the associated Monte Carlo simulation. Special attention is given to the symmetric Møller/Bhabha detector system, which is not described in full elsewhere. The results achieved by the OLYMPUS collaboration, reported herein, achieve unprecedented precision over a wide kinematic range and make a significant contribution to the understanding of two-photon exchange.
We present a comprehensive study of two-boson exchange (TBE) corrections in parity-violating electron-proton elastic scattering. Within a hadronic framework, we compute contributions from box (and crossed box) diagrams in which the intermediate states are described by nucleons and Delta baryons. The Delta contribution is found to be much smaller than the nucleon one at backward angles (small epsilon), but becomes dominant in the forward scattering limit (epsilon near 1), where the nucleon contribution vanishes. The dependence of the corrections on the input hadronic form factors is small for Q^2
The parity-conserving single-spin beam asymmetry of elastic electron-proton scattering is induced by an absorptive part of the two-photon exchange amplitude. We demonstrate that this asymmetry has logarithmic and double-logarithmic enhancement due to contributions of hard collinear quasi-real photons. An optical theorem is used to evaluate the asymmetry in terms of the total photoproduction cross section on the proton.
