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

Rimal, D.; Adikaram, D.; Raue, B. A.; Weinstein, L. B.; Arrington, J.; Brooks, W. K.; Ungaro, M.; Adhikari, K. P.; Afanasev, A.; Akbar, Z.; Pereira, S. Anefalos; Badui, R.; Ball, J.; Baltzell, N. A.; Battaglieri, M.; Batourine, V.; Bedlinskiy, I.; Biselli, A.; Boiarinov, S.; Briscoe, W. J.; Bültmann, S.; Burkert, Volker; Carman, D. S.; Celentano, A.; Chetry, T.; Ciullo, G.; Clark, L.; Colaneri, L.; Cole, P. L.; Compton, N.; Contalbrigo, M.; Cortes, O.; Credé, V.; D’Angelo, A.; Dashyan, N.; Vita, R. De; Deur, A.; Djalali, C.; Dupré, R.; Egiyan, H.; Alaoui, A. El; Fassi, L. El; Eugenio, P.; Fanchini, E.; Fedotov, G.; Fersch, R.; Filippi, A.; Fleming, J. A.; Forest, Tony A.; Fradi, A.; Gevorgyan, N.; Ghandilyan, Y.; Gilfoyle, G. P.; Giovanetti, K. L.; Girod, F. X.; Gleason, C.; Gohn, W.; Golovatch, E.; Gothe, R. W.; Griffioen, K. A.; Guo, L.; Hafidi, K.; Hanretty, C.; Harrison, N.; Hattawy, M.; Heddle, D.; Hicks, K.; Holtrop, M.; Hughes, S. M.; Ilieva, Y.; Ireland, D. G.; Ишханов, Б. С.; Isupov, E. L.; Jenkins, D.; Jiang, Hao; Joosten, S.; Keller, D.; Khachatryan, G.; Khandaker, M.; Kim, W.; Klein, A.; Klein, F. J.; Kubarovsky, V.; Kuhn, S. E.; Kuleshov, S.; Lanza, L.; Lenisa, P.; Livingston, K.; Lu, H. Y.; MacGregor, I. J. D.; Markov, N.; McKinnon, B.; Mestayer, M. D.; Mirazita, M.; Mokeev, V.; Movsisyan, A.; Munévar, E.; Camacho, C. Muñoz; Nadel-Turoński, P.; Ni, A.; Niccolai, S.; Niculescu, G.; Niculescu, I.; Osipenko, M.; Ostrovidov, A. I.; Paolone, M.; Paremuzyan, R.; Park, K.; Pasyuk, E.; Phelps, W.; Pisano, S.; Pogorelko, O.; Price, J. W.; Prok, Y.; Protopopescu, D.; Puckett, A. J. R.; Rizzo, A.; Rosner, G.; Rossi, P.; Roy, P.; Sabatié, F.; Salgado, C.; Schumacher, R. A.; Seder, E.; Sharabian, Y. G.; Skorodumina, Iu.; Smith, G. D.; Sokhan, D.; Sparveris, N.; Stanković, Ivana; Stepanyan, S.; Strauch, S.; Sytnik, V.; Taiuti, M.; Torayev, B.; Voskanyan, H.; Voutier, E.; Walford, N. K.; Watts, D. P.; Wei, Xin; Wood, M. H.; Zachariou, N.; Zana, L.; Zhang, J.; Zhao, Zhiwen; Zonta, I.

doi:10.1103/physrevc.95.065201

Cited by 49 publications

(26 citation statements)

References 77 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%

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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%

“…The latter allows one to predict the higher moments r 2n , which affect R when using Equation (12), to analyze electron scattering [116]. These higher moments are, however, very far from the values obtained from electron scattering [63].…”

Section: Vector Dominance Model Fitsmentioning

confidence: 99%

Proton Charge Radius from Electron Scattering

Sick

2017

Atoms

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“…In this paper I use three different global fits to unpolarized measurements to make predictions of the hard TPE effect necessary to resolve the form factor discrepancy. I compare these predictions to the results of the recent TPE experiments, at VEPP-3 [3], at CLAS [4,10], and the OLYMPUS Experiment [5]. I find that the spread in predictions from using different global fits is very large, arXiv:1907.07318v1 [nucl-ex] 17 Jul 2019…”

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confidence: 91%

How much two-photon exchange is needed to resolve the proton form factor discrepancy?

Schmidt¹

2020

J. Phys. G: Nucl. Part. Phys.

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Background: One possible explanation for the proton form factor discrepancy is a contribution to the elastic electron-proton cross section from hard two-photon exchange (TPE), a typically neglected radiative correction. Hard TPE cannot be calculated in a model-independent way, but it can be determined experimentally by looking for deviations from unity in the ratio of positron-proton to electron-proton cross sections. Three recent experiments have measured this cross section ratio to quantify hard TPE.Purpose: To interpret the results of these experiments, it is germane to ask: "How large of a deviation from unity is necessary to fully resolve the form factor discrepancy?"Method: I derive an expression using a minimal set of assumptions to answer this question based on global fits to unpolarized and polarized elastic scattering data.Results: I find wide variation when using different global fits, implying that the magnitude of the form factor discrepancy is not well-constrained.Conclusions: Recent measurements of hard TPE can easily accommodate the hypothesis that TPE underlies the proton form factor discrepancy.

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“…The CLAS Two-Photon Experiment [35][36][37] used a teritiary beam of electron/positron pairs. The Hall B secondary photon beam was directed on gold converter foil to create electron and positron pairs, which in turn were directed through a magnetic chicane so that the unconverted photons could be blocked.…”

Section: Clas Two-photon Experimentsmentioning

confidence: 99%

The continuing story of two-photon exchange: results from the OLYMPUS experiment

Schmidt¹

2017

Proceedings of 55th International Winter Meeting on Nuclear Physics — PoS(BORMIO2017)

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Over the past two decades, a discrepancy has emerged between two different techniques for measuring the proton's electromagnetic form factors. Unpolarized electron-proton cross section measurements paint a picture of the proton's internal structure that is incompatible with measurements from polarization transfer experiments. The leading hypothesis is that the discrepancy is caused by a typically neglected radiative correction, hard two-photon exchange (TPE), which would affect the two measurement techniques in different ways. There is no model independent way to calculate hard TPE, but it can be measured experimentally by looking for an asymmetry between the positron-proton and electron-proton elastic cross sections. Three recent experiments have attempted to quantify this asymmetry, and, just last month, the third of these, called OLYMPUS, released its results [1]. The OLYMPUS experiment collected data in 2012 at DESY, alternating between 2 GeV electron and positron beams, directed through a hydrogen gas target. The scattered lepton and recoiling proton were detected in coincidence with a large acceptance toroidal spectrometer. The relative luminosity between the two beam species was monitored with three independent systems, and the results comprise 3 inverse fb of integrated luminosity, exceeding by a factor of three the other two TPE experiments combined. In this talk, the case for the TPE hypothesis will be presented, the OLYMPUS experiment will be described, and the results of all three experiments will be compared.

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

Cited by 49 publications

References 77 publications

Proton Charge Radius from Electron Scattering

Proton Charge Radius from Electron Scattering

How much two-photon exchange is needed to resolve the proton form factor discrepancy?

The continuing story of two-photon exchange: results from the OLYMPUS experiment

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