Demonstration of a novel technique to measure two-photon exchange effects in elastic e±p scattering

Moteabbed, M. et al. (2013) Demonstration of a novel technique to measure two-photon exchange effects in elastic e±p scattering. Physical Review C, 88, 025210:1-025210:14. (doi:10.1103/PhysRevC.88.025210)

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Abstract

Background: The discrepancy between proton electromagnetic form factors extracted using unpolarized and polarized scattering data is believed to be a consequence of two-photon exchange (TPE) effects. However, the calculations of TPE corrections have significant model dependence, and there is limited direct experimental evidence for such corrections.<p></p> Purpose: The TPE contributions depend on the sign of the lepton charge in e±p scattering, but the luminosities of secondary positron beams limited past measurement at large scattering angle where the TPE effects are believe to be most significant. We present the results of a new experimental technique for making direct e±p comparisons, which has the potential to make precise measurements over a broad range in Q2 and scattering angles.<p></p> Methods: We use the Jefferson Lab electron beam and the Hall B photon tagger to generate a clean but untagged photon beam. The photon beam impinges on a converter foil to generate a mixed beam of electrons, positrons, and photons. A chicane is used to separate and recombine the electron and positron beams while the photon beam is stopped by a photon blocker. This provides a combined electron and positron beam, with energies from 0.5 to 3.2 GeV, which impinges on a liquid hydrogen target. The large acceptance CLAS detector is used to identify and reconstruct elastic scattering events, determining both the initial lepton energy and the sign of the scattered lepton.<p></p> Results: The data were collected in two days with a primary electron beam energy of only 3.3 GeV, limiting the data from this run to smaller values of Q2 and scattering angle. Nonetheless, this measurement yields a data sample for e±p with statistics comparable to those of the best previous measurements. We have shown that we can cleanly identify elastic scattering events and correct for the difference in acceptance for electron and positron scattering. Because we ran with only one polarity for the chicane, we are unable to study the difference between the incoming electron and positron beams. This systematic effect leads to the largest uncertainty in the final ratio of positron to electron scattering: R= 1.027±0.005±0.05 for <Q2>= 0.206 GeV2 and 0.830≥ǫ≥0.943.<p></p> Conclusions: We have demonstrated that the tertiary e± beam generated using this novel technique provides the opportunity for dramatically improved comparisons of e±p scattering, covering a significant range in both Q2 and scattering angle. Combining data with different chicane polarities will allow for detailed studies of the difference between the incoming e+ and e− beams.<p></p>

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 Montgomery, Dr Rachel and Ireland, Professor David and Lewis, Dr Stefanie and Rosner, Professor Guenther
Authors: Moteabbed, M., Niroula, M., Raue, B.A., Weinstein, L.B., Adikaram, D., Arrington, J., Brooks, W.K., Lachniet, J., Rimal, D., Ungaro, M., Afanasev, A., Adhikari, K.P., Aghasyan, M., Amaryan, M.J., Anefalos Pereira, S., Avakian, H., Ball, J., Baltzell, N.A., Battaglieri, M., Batourine, V., Bedlinskiy, I., Bennett, R.P., Biselli, A.S., Bono, J., Boiarinov, S., Briscoe, W.J., Burkert, V.D., Carman, D.S., Celentano, A., Chandavar, S., Cole, P.L., Collins, P., Contalbrigo, M., Cortes, O., Crede, V., D'Angelo, A., Dashyan, N., De Vita, R., De Sanctis, E., Deur, A., Djalali, C., Doughty, D., Dupre, R., Egiyan, H., Fassi, L.E., Eugenio, P., Fedotov, G., Fegan, S., Fersch, R., Fleming, J.A., Gevorgyan, N., Gilfoyle, G.P., Giovanetti, K.L., Girod, F.X., Goetz, J.T., Gohn, W., Golovatch, E., Gothe, R.W., Griffioen, K.A., Guidal, M., Guler, N., Guo, L., Hafidi, K., Hakobyan, H., Hanretty, C., Harrison, N., Heddle, D., Hicks, K., Ho, D., Holtrop, M., Hyde, C.E., Ilieva, Y., Ireland, D.G., Ishkhanov, B.S., Isupov, E.L., Jo, H.S., Joo, K., Keller, D., Khandaker, M., Kim, A., Klein, F.J., Koirala, S., Kubarovsky, A., Kubarovsky, V., Kuhn, S. E., Kuleshov, S.V., Lewis, S., Lu, H.Y., MacCormick, M., MacGregor, I.J.D., Martinez, D., Mayer, M., McKinnon, B., Mineeva, T., Mirazita, M., Mokeev, V., Montgomery, R.A., Moriya, K., Moutarde, H., Munevar, E., Munoz Camacho, C., Nadel-Turonski, P., Nasseripour, R., Niccolai, S., Niculescu, G., Niculescu, I., Osipenko, M., Ostrovidov, A.I., Pappalardo, L.L., Paremuzyan, R., Park, K., Park, S., Phelps, E., Phillips, J.J., Pisano, S., Pogorelko, O., Pozdniakov, S., Price, J.W., Procureur, S., Protopopescu, D., Puckett, A.J.R., Ripani, M., Rosner, G., Rossi, P., Sabatié, F., Saini, M. S., Salgado, C., Schott, D., Schumacher, R. A., Seder, E., Seraydaryan, H., Sharabian, Y.G., Smith, E.S., Smith, G.D., Sober, D.I., Sokhan, D., Stepanyan, S., Strauch, S., Tang, W., Taylor, C.E., Tian, Y., Tkachenko, S., Voskanyan, H., Voutier, E., Walford, N.K., 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:CLAS
Journal Name:Physical Review C
ISSN:0556-2813
ISSN (Online):1089-490X

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