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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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
Glasgow Author(s) Enlighten ID:Sokhan, Dr Daria and Fegan, Mr Stuart and Protopopescu, Dr Dan and Smith, Mr Gary and MacGregor, Professor Douglas 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 (Online):1079-7114
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