Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments showa significant disagreement that grows with the squared four-momentum transfer (Q(2)). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose: We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6-GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron-positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm-long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons, we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. Results: We present previously unpublished results for the quantity R-2 gamma, the TPE correction to the elastic-scattering cross section, at Q(2) approximate to 0.85 and 1.45 GeV2 over a large range of virtual photon polarization epsilon. Conclusions: Our results, along with recently published results from VEPP-3, demonstrate a nonzero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q(2) approximate to 2 GeV2. These data are consistent with an increase in R-2 gamma. with decreasing e at Q(2) approximate to 0.85 and 1.45 GeV2. There are indications of a slight increase in R-2 gamma with Q(2).

Rimal, D., Adikaram, D., Raue, B.a., Weinstein, L.b., Arrington, J., Brooks, W.k., et al. (2017). Measurement of two-photon exchange effect by comparing elastic e±p cross sections. PHYSICAL REVIEW C, 95(6), 065201-1-065201-20 [10.1103/PhysRevC.95.065201].

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

Colaneri L.;D'Angelo A.;Pisano S.;Rizzo A.;Zonta I.
2017

Abstract

Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments showa significant disagreement that grows with the squared four-momentum transfer (Q(2)). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose: We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6-GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron-positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm-long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons, we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. Results: We present previously unpublished results for the quantity R-2 gamma, the TPE correction to the elastic-scattering cross section, at Q(2) approximate to 0.85 and 1.45 GeV2 over a large range of virtual photon polarization epsilon. Conclusions: Our results, along with recently published results from VEPP-3, demonstrate a nonzero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q(2) approximate to 2 GeV2. These data are consistent with an increase in R-2 gamma. with decreasing e at Q(2) approximate to 0.85 and 1.45 GeV2. There are indications of a slight increase in R-2 gamma with Q(2).
Pubblicato
Rilevanza internazionale
Articolo
Esperti anonimi
Settore FIS/04 - Fisica Nucleare e Subnucleare
English
Con Impact Factor ISI
CLAS Collaboration
https://journals.aps.org/prc/pdf/10.1103/PhysRevC.95.065201
Rimal, D., Adikaram, D., Raue, B.a., Weinstein, L.b., Arrington, J., Brooks, W.k., et al. (2017). Measurement of two-photon exchange effect by comparing elastic e±p cross sections. PHYSICAL REVIEW C, 95(6), 065201-1-065201-20 [10.1103/PhysRevC.95.065201].
Rimal, D; Adikaram, D; Raue, Ba; Weinstein, Lb; Arrington, J; Brooks, Wk; Ungaro, M; Adhikari, Kp; Afanasev, Av; Akbar, Z; Pereira, Sa; Badui, Ra; Ball, J; Baltzell, Na; Battaglieri, M; Batourine, V; Bedlinskiy, I; Biselli, As; Boiarinov, S; Briscoe, Wj; Bultmann, S; Burkert, Vd; Carman, Ds; Celentano, A; Chetry, T; Ciullo, G; Clark, L; Colaneri, L; Cole, Pl; Compton, N; Contalbrigo, M; Cortes, O; Crede, V; D'Angelo, A; Dashyan, N; De Vita, R; Deur, A; Djalali, C; Dupre, R; Egiyan, H; Alaoui, Ae; Fassi, Le; Eugenio, P; Fanchini, E; Fedotov, G; Fersch, R; Filippi, A; Fleming, Ja; Forest, Ta; Fradi, A; Gevorgyan, N; Ghandilyan, Y; Gilfoyle, Gp; Giovanetti, Kl; Girod, Fx; Gleason, C; Gohn, W; Golovatch, E; Gothe, Rw; Griffioen, Ka; Guo, L; Hafidi, K; Hanretty, C; Harrison, N; Hattawy, M; Heddle, D; Hicks, K; Holtrop, M; Hughes, Sm; Ilieva, Y; Ireland, Dg; Ishkhanov, Bs; Isupov, El; Jenkins, D; Jiang, H; Joosten, S; Keller, D; Khachatryan, G; Khandaker, M; Kim, W; Klein, A; Klein, Fj; Kubarovsky, V; Kuhn, Se; Kuleshov, Sv; Lanza, L; Lenisa, P; Livingston, K; Lu, Hy; Macgregor, Ijd; Markov, N; Mckinnon, B; Mestayer, Md; Mirazita, M; Mokeev, V; Movsisyan, A; Munevar, E; Camacho, Cm; Nadel-Turonski, P; Ni, A; Niccolai, S; Niculescu, G; Niculescu, I; Osipenko, M; Ostrovidov, Ai; Paolone, M; Paremuzyan, R; Park, K; Pasyuk, E; Phelps, W; Pisano, S; Pogorelko, O; Price, Jw; Prok, Y; Protopopescu, D; Puckett, Ajr; Rizzo, A; Rosner, G; Rossi, P; Roy, P; Sabatie, F; Salgado, C; Schumacher, Ra; Seder, E; Sharabian, Yg; Skorodumina, I; Smith, Gd; Sokhan, D; Sparveris, N; Stankovic, I; Stepanyan, S; Strauch, S; Sytnik, V; Taiuti, M; Torayev, B; Voskanyan, H; Voutier, E; Walford, Nk; Watts, Dp; Wei, X; Wood, Mh; Zachariou, N; Zana, L; Zhang, J; Zhao, Zw; Zonta, I
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2108/194467
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