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Author: Publisher: ISBN: Category : Languages : en Pages : 89
Book Description
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
Author: Publisher: ISBN: Category : Languages : en Pages : 89
Book Description
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
Author: Rebecca Lynn Russell Publisher: ISBN: Category : Languages : en Pages : 190
Book Description
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
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
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.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Recent theoretical developments in the studies of two-photon exchange effects in elastic electron-proton scattering are reviewed. Two-photon exchange mechanism is considered a likely source of discrepancy between polarized and unpolarized experimental measurements of the proton electric form factor at momentum transfers of several GeV$^2$. This mechanism predicts measurable effects that are currently studied experimentally.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer by using a quark-parton representation of virtual Compton scattering. We thus can relate the two-photon exchange amplitude to the generalized parton distributions which also enter in other wide angle scattering processes. We find that the interference of one- and two-photon exchange contribution is able to substantially resolve the difference between electric form factor measurements from Rosenbluth and polarization transfer experiments. Two-photon exchange has additional consequences which could be experimentally observed, including nonzero polarization effects and a positron-proton/electron-proton scattering asymmetry. The predicted Rosenbluth plot is no longer precisely linear; it acquires a measurable curvature, particularly at large laboratory angle.
Author: W. Melnitchouk Publisher: ISBN: Category : Languages : en Pages :
Book Description
We present an explicit quantum field theoretical calculation of two-photon exchange contribution to elastic electron-proton scattering and to Delta resonance production in electron-proton collisions.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
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 the angular dependence compatible with both the polarization transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors.
Author: Publisher: ISBN: Category : Languages : en Pages : 13
Book Description
The discrepancy between proton electromagnetic form factors extracted using unpolarized and polarized scattering data is believed to be a consequence of two-photon exchange (TPE) effects. However, the calculations of TPE corrections have significant model dependence, and there is limited direct experimental evidence for such corrections. The TPE contributions depend on the sign of the lepton charge in e"p scattering, but the luminosities of secondary positron beams limited past measurement at large scattering angles, where the TPE effects are believe to be most significant. We present the results of a new experimental technique for making direct e"p comparisons, which has the potential to make precise measurements over a broad range in Q2 and scattering angles. We use the Jefferson Laboratory electron beam and the Hall B photon tagger to generate a clean but untagged photon beam. The photon beam impinges on a converter foil to generate a mixed beam of electrons, positrons, and photons. A chicane is used to separate and recombine the electron and positron beams while the photon beam is stopped by a photon blocker. This provides a combined electron and positron beam, with energies from 0.5 to 3.2 GeV, which impinges on a liquid hydrogen target. The large acceptance CLAS detector is used to identify and reconstruct elastic scattering events, determining both the initial lepton energy and the sign of the scattered lepton. The data were collected in two days with a primary electron beam energy of only 3.3 GeV, limiting the data from this run to smaller values of Q2 and scattering angle. Nonetheless, this measurement yields a data sample for e"p with statistics comparable to those of the best previous measurements. We have shown that we can cleanly identify elastic scattering events and correct for the difference in acceptance for electron and positron scattering. Because we ran with only one polarity for the chicane, we are unable to study the difference between the incoming electron and positron beams. This systematic effect leads to the largest uncertainty in the final ratio of positron to electron scattering: R=1.027"0.005"0.05 for Q2=0.206 GeV2 and 0.830 d"[epsilon] d"0.943. We have demonstrated that the tertiary e" beam generated using this technique provides the opportunity for dramatically improved comparisons of e"p scattering, covering a significant range in both Q2 and scattering angle. Combining data with different chicane polarities will allow for detailed studies of the difference between the incoming e+ and e- beams.
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Author: Rebecca Lynn Russell
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Author: W. Melnitchouk
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Author: Gary Kalman Greenhut
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