Flavor decomposition of the nucleon electromagnetic form factors at low<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>Q</mml:mi><mml:mn>2</mml:mn></mml:msup></mml:math>

Qattan, I. A.; Arrington, J.; Alsaad, Ahmad

doi:10.1103/physrevc.91.065203

Cited by 27 publications

(24 citation statements)

References 109 publications

(145 reference statements)

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“…For Q 2 above 3-4 GeV 2 , an ε-dependent correction of approximately 5% could explain the observed discrepancy [13,14,17,40]. At Q 2 < 2 GeV 2 the discrepancy is smaller and provides a less sensitive constraint on TPE contributions [41], though it is consistent with a few % correction.…”

Section: Introductionsupporting

confidence: 57%

Measurement of two-photon exchange effect by comparing elastic e±p cross sections

Rimal¹,

Adikaram²,

Raue³

et al. 2017

Phys. Rev. C

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Measurement of two-photon exchange effect by comparing elastic e ± p cross sections Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four momentum transfer (Q 2 ). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose:We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy.Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6 GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron/positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented.Results: We present previously unpublished results for the quantity R2γ, the TPE correction to the elasticscattering cross section, at Q 2 ≈ 0.85 and 1.45 GeV 2 over a large range of virtual photon polarization ε. Conclusions:Our results, along with recently published results from VEPP-3, demonstrate a non-zero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q 2 ≈ 2 GeV 2 . These data are consistent with an increase in R2γ with decreasing ε at Q 2 ≈ 0.85 and 1.45 GeV 2 . There are indications of a slight increase in R2γ with Q 2 .

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Section: Introductionsupporting

confidence: 57%

Measurement of two-photon exchange effect by comparing elastic e±p cross sections

Rimal¹,

Adikaram²,

Raue³

et al. 2017

Phys. Rev. C

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“…[10] is in best agreement with the data , however it tends toward overestimating the data. Phenomenological extractions of TPE contributions in this Q 2 range [11] tend to predict somewhat larger effects than the hadronic calculation, but such extractions are not as much reliable at these Q 2 values, where the discrepancy between Rosenbluth and polarization results is not clear.…”

Section: Preliminary Results and Conclusionmentioning

confidence: 77%

Two-photon exchange contribution in elastic electron-proton scattering, experiment at the VEPP-3 storage ring

Nikolenko

Arrington

Барков

et al. 2014

EPJ Web of Conferences

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Abstract. We describe a precise measurement of the ratio of the (e + p) to (e − p) elastic scattering cross sections. This comparison is sensitive to the effect of two-photon exchange contributions which may be the cause for inconsistent extractions of the proton form factors obtained using different methods. The experiment was performed at storage ring VEPP-3, Novosibirsk at energies of positron/electron beams of 1.0 and 1.6 GeV with electron/positron scattering angles θ = 65÷105• for the first case and 15÷25• and 55÷75• for the second case. Details of the experiment and the preliminary results are presented.

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“…[66,68] yielded χ 2 ν > 1.80. I also determined the values of (G p E (Q 2 )) 2 using the improved parametrization of the ratio R and its associated uncertainty [52]. The values of (G p E /G D (Q 2 )) 2 and (G p M /µ p G D (Q 2 )) 2 along with the fit results for the TPE amplitudes coefficients are included in the online Supplemental Material [62].…”

Section: Resultsmentioning

confidence: 99%

Phenomenological extraction of two-photon exchange amplitudes from elastic electron-proton scattering cross section data

Qattan

2017

Phys. Rev. C

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Background: The inconsistency in the results obtained from the Rosenbluth separation method and the high-Q 2 recoil polarization results on the ratio µpG p E /G p M implies a systematic difference between the two techniques. Several studies suggested that missing higher order radiative corrections to elastic electron-proton scattering cross section σR(ε, Q 2 ) and in particular hard two-photon-exchange (TPE) effect contributions could account for the discrepancy.Purpose: In this work, I improve on and extend to low-and high-Q 2 values the extractions of the ε dependence of the real parts of the TPE amplitudes, relative to the magnetic form factor, as well as the ratio P l /P Born l (ε, Q 2 ) using world data on σR(ε, Q 2 ) with an emphasis on precise new data covering the low-momentum region which is sensitive to the large-scale structure of the nucleon.Method: I combine cross section and polarization measurements of elastic electron-proton scattering to extract the TPE amplitudes. Because the recoil polarization data were confirmed "experimentally" to be essentially independent of ε, I constrain the ratio Pt/P l (ε, Q 2 ) to its ε-independent term (Born value) by setting the TPE contributions to zero. That allows for the amplitude YM (ε, Q 2 ) and σR(ε, Q 2 ) to be expressed in terms of the remaining two amplitudes YE(ε, Q 2 ) and Y3(ε, Q 2 ) which in turn were parametrized as second-order polynomials in ε and Q 2 to reserve as possible the linearity of σR(ε, Q 2 ) as well as to account for possible nonlinearities in the TPE amplitudes. Further, I impose the Regge limit which ensures the vanishing of the TPE contributions to σR(ε, Q 2 ) and the TPE amplitudes in the limit ε → 1.Results: I provide simple parametrizations of the TPE amplitudes, along with an estimate of the fit uncertainties. The extracted TPE amplitudes are compared to previous phenomenological extractions and TPE calculations. The P l /P Born l ratio is extracted using the new parametrizations of the TPE amplitudes and compared to previous extractions, TPE calculations, and direct measurements at Q 2 = 2.50 (GeV/c) 2 . Conclusions:The extracted TPE amplitudes are on the few-percentage-points level, and behave roughly linearly with increasing Q 2 where they become nonlinear at high Q 2 . On the contrary to YM which is influenced mainly by elastic contributions, I find YE to be influenced by inelastic contributions at large Q 2 values. While YE and Y3 differ in magnitude, they have opposite sign and tend to partially cancel each other. This suggests that the TPE correction to σR(ε, Q 2 ) is driven mainly by YM and to a lesser extent by Y3 in agreement with previous phenomenological extractions and hadronic TPE calculations.

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Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

Cited by 27 publications

References 109 publications

Measurement of two-photon exchange effect by comparing elastic e±p cross sections

Measurement of two-photon exchange effect by comparing elastic e±p cross sections

Two-photon exchange contribution in elastic electron-proton scattering, experiment at the VEPP-3 storage ring

Phenomenological extraction of two-photon exchange amplitudes from elastic electron-proton scattering cross section data

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