Measurement of the Two-Photon Exchange Contribution to the Elastic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>e</mml:mi><mml:mo>±</mml:mo></mml:msup><mml:mi>p</mml:mi></mml:math>Scattering Cross Sections at the VEPP-3 Storage Ring

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

doi:10.1103/physrevlett.114.062005

Cited by 107 publications

(110 citation statements)

References 40 publications

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“…The main effect of the latter is to remove the discrepancy between values of G e (q) at very large q resulting from determinations via Rosenbluth separation and polarization transfer, respectively. For a recent review, see [11]; for experiments checking on the two-photon exchange, see [12][13][14].…”

Section: Electron Scatteringmentioning

confidence: 99%

Proton Charge Radius from Electron Scattering

Sick

2017

Atoms

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Abstract:The rms-radius R of the proton charge distribution is a fundamental quantity needed for precision physics. This radius, traditionally determined from elastic electron-proton scattering via the slope of the Sachs form factor G e (q 2 ) extrapolated to momentum transfer q 2 = 0, shows a large scatter. We discuss the approaches used to analyze the e-p data, partly redo these analyses in order to identify the sources of the discrepancies and explore alternative parameterizations. The problem lies in the model dependence of the parameterized G(q) needed for the extrapolation. This shape of G(q < q min ) is closely related to the shape of the charge density ρ(r) at large radii r, a quantity that is ignored in most analyses. When using our physics knowledge about this large-r density together with the information contained in the high-q data, the model dependence of the extrapolation is reduced, and different parameterizations of the pre-2010 data yield a consistent value for R = 0.887 ± 0.012 fm. This value disagrees with the more precise value 0.8409 ± 0.0004 fm determined from the Lamb shift in muonic hydrogen.

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Section: Electron Scatteringmentioning

confidence: 99%

Proton Charge Radius from Electron Scattering

Sick

2017

Atoms

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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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“…In summary, most of these studies can explain parts of the difference in terms of TPE effects, although the precise interplay between hadronic and partonic effects remains an open question. To this end, the ratio of the elastic e + p/e − p cross sections has been measured by several experiments to provide a definitive test of the magnitude of TPE effects [667][668][669][670].…”

Section: Overview Of Two-photon Physicsmentioning

confidence: 99%

Baryons as relativistic three-quark bound states

Eichmann

Sanchis-Alepuz

Williams

et al. 2016

Progress in Particle and Nuclear Physics

403

503

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We review the spectrum and electromagnetic properties of baryons described as relativistic three-quark bound states within QCD. The composite nature of baryons results in a rich excitation spectrum, whilst leading to highly non-trivial structural properties explored by the coupling to external (electromagnetic and other) currents. Both present many unsolved problems despite decades of experimental and theoretical research. We discuss the progress in these fields from a theoretical perspective, focusing on nonperturbative QCD as encoded in the functional approach via Dyson-Schwinger and Bethe-Salpeter equations. We give a systematic overview as to how results are obtained in this framework and explain technical connections to lattice QCD. We also discuss the mutual relations to the quark model, which still serves as a reference to distinguish 'expected' from 'unexpected' physics. We confront recent results on the spectrum of non-strange and strange baryons, their form factors and the issues of two-photon processes and Compton scattering determined in the DysonSchwinger framework with those of lattice QCD and the available experimental data. The general aim is to identify the underlying physical mechanisms behind the plethora of observable phenomena in terms of the underlying quark and gluon degrees of freedom.

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Measurement of the Two-Photon Exchange Contribution to the Elastice±pScattering Cross Sections at the VEPP-3 Storage Ring

Cited by 107 publications

References 40 publications

Proton Charge Radius from Electron Scattering

Proton Charge Radius from Electron Scattering

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

Baryons as relativistic three-quark bound states

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