Towards a resolution of the proton form factor problem: new electron and positron scattering data

Adikaram, D. et al. (2015) Towards a resolution of the proton form factor problem: new electron and positron scattering data. Physical Review Letters, 114, 062003. (doi:10.1103/PhysRevLett.114.062003)

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Publisher's URL: http://dx.doi.org/10.1103/PhysRevLett.114.062003

Abstract

There is a significant discrepancy between the values of the proton electric form factor, G_E^p, extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of G_E^p from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization (\varepsilon) and momentum transfer (Q^2) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing \varepsilon at Q^2 = 1.45 \text{ GeV}^2. This measurement is consistent with the size of the form factor discrepancy at Q^2\approx 1.75 GeV<sup>2</sup> and with hadronic calculations including nucleon and \Delta intermediate states, which have been shown to resolve the discrepancy up to 2-3 GeV<sup>2</sup>.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Sokhan, Dr Daria and Fegan, Mr Stuart and Protopopescu, Dr Dan and Smith, Mr Gary and MacGregor, Professor I and McKinnon, Dr Bryan and Livingston, Dr Kenneth and Montgomery, Dr Rachel and Anderson, Mr Mark and Ireland, Professor David and Rosner, Professor Guenther
Authors: Adikaram, D., Rimal, D., Weinstein, L. B., Raue, B., Khetarpal, P., Bennet, R. P., Arrington, J., Brooks, W. K., Adhikari, K. P., Afanasev, A. V., Amaryan, M. J., Anderson, M. D., Ball, J., Battaglieri, M., Bedlinskiy, I., Biselli, A. S., Bono, J., Boiarinov, S., Briscoe, W. J., Burkert, V. D., Carman, D. S., Celentano, A., Chandavar, S., Charles, G., Colaneri, L., Cole, P. L., Contalbrigo, M., D'Angelo, A., Dashyan, N., De Vita, R., De Sanctis, E., Deur, A., Djalali, C., Dodge, G. E., Dupre, R., Egiyan, H., El Alaoui, A., El Fassi, L., Eugenio, P., Fedotov, G., Fegan, S., Filippi, A., Fleming, J. A., Fradi, A., Gilfoyle, G. P., Giovanetti, K. L., Girod, F. X., Goetz, J. T., Gohn, W., Golovatch, E., Gothe, R. W., Griffioen, K. A., Guidal, M., Guo, L., Hafidi, K., Hakobyan, H., Harrison, N., Hattawy, M., Hicks, K., Holtrop, M., Hughes, S. M., Hyde, C. E., Ilieva, Y., Ireland, D.G., Ishkhanov, B. S., Jenkins, D., Jiang, H., Joo, K., Joosten, S., Khandaker, M., Kim, W., Klein, A., Klein, F. J., Koirala, S., Kubarovsky, V., Kuhn, S. E., Livingston, K., Lu, H. Y., MacGregor, I. J. D., Markov, N., Mayer, M., McKinnon, B., Mestayer, M. D., Meyer, C. A., Mirazita, M., Mokeev, V., Montgomery, R., Moody, C. I., Moutarde, H., Movsisyan, A., Munoz Camacho, C., Nadel-Turonski, P., Niccolai, S., Niculescu, G., Osipenko, M., Ostrovidov, A. I., Park, K., Pasyuk, E., Pisano, S., Pogorelko, O., Procureur, S., Prok, Y., Protopopescu, D., Puckett, A. J. R., Ripani, M., Rizzo, A., Rosner, G., Rossi, P., Sabatie, F., Schott, D., Schumacher, R. A., Sharabian, Y. G., Simonyan, A., Skorodumina, I., Smith, E. S., Smith, G. D., Sober, D. I., Sokhan, D., Sparveris, N., Stepanyan, S., Strauch, S., Sytnik, V., Taiuti, M., Tian, Y., Trivedi, A., Ungaro, M., Voskanyan, H., Watts, D. P., Voutier, E., Walford, N. K., Wei, X., Wood, M. H., Zachariou, N., Zana, L., Zhang, J., Zhao, Z. W., and Zonta, I.
Subjects:Q Science > QC Physics
College/School:College of Science and Engineering > School of Physics and Astronomy
Research Group:Nuclear Physics
Journal Name:Physical Review Letters
Publisher:American Physical Society
ISSN:0031-9007
ISSN (Online):1079-7114
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