Proton form factors and two-photon exchange in elastic electron-proton scattering

Nikolenko, D. M.; Arrington, J.; Барков, Л.М.; Vries, H. de; Gauzshtein, V. V.; Golovin, R. A.; Gramolin, A. V.; Дмитриев, В. Ф.; Zhilich, V.; Zevakov, S. A.; Kaminsky, V.V.; Lazarenko, B. A.; Mishnev, S. I.; Muchnoi, N. Yu.; Neufeld, V. V.; Rachek, I. A.; Sadykov, R. Sh.; Stibunov, V. N.; Toporkov, D. K.; Holt, R. J.; Shestakov, Yu. V.

doi:10.1134/s1063778815020234

Cited by 6 publications

(4 citation statements)

References 56 publications

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“…The extent to which two-photon exchange (TPE) affects the extraction of R E from the MAMI data has been debated in the literature [5,8,9,12,18,[26][27][28][29][30][31][32]. The cross sections given in Ref.…”

Section: Fig 2 (Color Online)mentioning

confidence: 99%

Evaluation of the strength of electron-proton scattering data for determining the proton charge radius

Horbatsch

Hessels

2016

Phys. Rev. C

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Precisely measured electron-proton elastic scattering cross sections [Phys. Rev. Lett. 105, 242002 (2010)] are reanalyzed to evaluate their strength for determining the rms charge radius (RE) of the proton. More than half of the cross sections at lowest Q 2 are fit using two single-parameter formfactor models, with the first based on a dipole parametrization, and the second on a linear fit to a conformal-mapping variable. These low-Q 2 fits extrapolate the slope of the form factor to Q 2 =0 and determine RE values of approximately 0.84 and 0.89 fm, respectively. Fits spanning all Q 2 , in which the single constants are replaced with cubic splines at larger Q 2 , lead to similar results for RE. We conclude that the scattering data is consistent with RE ranging from at least 0.84 to 0.89 fm, and therefore cannot resolve the discrepancy between determinations of RE made using muonic and electronic hydrogen-atom spectroscopy. PACS numbers: \pacs{06.20. Jr,13.40.Gp,14.20.Dh,25.30.Bf} Recent measurements of the n=2 energy intervals of muonic hydrogen, when compared to precise QED theory for this exotic atom, lead to a determination [1,2] of the rms charge radius of the proton (R E ) of 0.84087(39) fm. This value disagrees by 4.5 standard deviations with a value of 0.8758(77) fm obtained from a similar comparison [3] between QED theory and several precision measurements in the ordinary hydrogen atom. A third determination of R E can be obtained from precise measurements of the the cross sections for elastic scattering between electrons and protons. The most precise electronproton scattering experiment is the recent measurement [4] of the MAMI collaboration, and their analysis [5] leads to R E = 0.879(8) fm, which disagrees with the muonic hydrogen value by 4.6 standard deviations.CODATA [3] uses a combination of the scattering and hydrogen values to obtain R E , and its value differs from the muonic hydrogen value by 7 standard deviations. This disagreement has now widely been referred to as the proton size puzzle [6]. Many papers have discussed this puzzle, including many that have proposed physics beyond the standard model [7].Because of the importance of the electron-proton scattering data to this puzzle, the data has been extensively scrutinized and discussed [5,[8][9][10][11][12][13][14][15][16][17][18][19][20]. The present work reanalyzes the scattering data and concludes that it is consistent with a much larger range of R E values than obtained by others. This range makes it consistent with both the hydrogen and muonic hydrogen determinations of R E , therefore removing one component of the proton radius puzzle.Our analysis uses two separate simple one-parameter form-factor models. Both of these simple models fit well to the low-Q 2 data, but the two give discrepant values for R E . Since neither model can be ruled out, the uncertainty in R E must, at minimum, be expanded to en- * hessels@yorku.ca compass both values. Generalizations of both models fit well to the entire MAMI data set, and give similarly dis...

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Section: Fig 2 (Color Online)mentioning

confidence: 99%

Evaluation of the strength of electron-proton scattering data for determining the proton charge radius

Horbatsch

Hessels

2016

Phys. Rev. C

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“…The experiment, published in Ref. [32,35], was performed at the VEPP-3 storage ring. The e ± p cross section was measured for two beam energies, 1.6 and 1 GeV and different lepton scattering angles, spanning such ǫ, Q 2 kinematical ranges: 0.272 < ǫ < 0.932, and 0.298…”

Section: A the Vepp Experimentsmentioning

confidence: 99%

Updated analysis of recent results on electron and positron elastic scattering on the proton

Tomasi–Gustafsson

2019

Phys. Rev. C

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We discuss recent experimental results concerning the cross section ratio of positron over electron elastic scattering on protons, and compare with the predictions of a pre-existent calculation. The deviation from unity of this ratio, i.e., a charge asymmetry different from zero, is the signature of contributions beyond the Born approximation. After reviewing the published results, we compare the elastic data to a calculation which includes the diagram corresponding to two-photon exchange.It turns out that all the data on the cross section ratio, in the limit of their precision, do not show evidence of enhanced two-photon contribution beyond the expected percent level. Our results confirm that experimental evidence for a large contribution of two-photon exchange is not yet found.

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“…1a. New measurements were proposed at VEPP-3, Novosibirsk [36,37], at CLAS (JLab) [38], and at Olympus (DESY) [39]. The results show that an asymmetry between electron and positron scattering exists indeed, and may reach 6-7%, but most of the asymmetry comes from the interference between initial and final photon emission, and is highly reduced when the data are properly radiatively corrected.…”

Section: Recent Literaturementioning

confidence: 99%

Two-Photon Exchange: Myth and History

Tomasi–Gustafsson

Pacetti

2018

Few-Body Syst

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After recalling the arguments for possible excess of two-photon contribution over α-counting, model independent statements about the consequences on the observables will be given. The relevant experimental data are discussed: (polarized and unpolarized) electron and positron elastic scattering on the proton, as well as annihilation data. A reanalysis of unpolarized electron-proton elastic scattering data is presented in terms of the electric to magnetic form factor squared ratio. This observable is in principle more robust against experimental correlations and global normalizations. The present analysis shows indeed that it is a useful quantity that contains reliable and coherent information. The comparison with the ratio extracted from the measurement of the longitudinal to transverse polarization of the recoil proton in polarized electron-proton scattering shows that the results are compatible. These results bring a decisive piece of information in the controversy on the deviation of the proton form factors from the dipole dependence.

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Proton form factors and two-photon exchange in elastic electron-proton scattering

Cited by 6 publications

References 56 publications

Evaluation of the strength of electron-proton scattering data for determining the proton charge radius

Evaluation of the strength of electron-proton scattering data for determining the proton charge radius

Updated analysis of recent results on electron and positron elastic scattering on the proton

Two-Photon Exchange: Myth and History

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