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Measurement of two-photon exchange effect by comparing elastic e±p cross sections
Journal
Physical Review C
Date Issued
2017-06-01
Author(s)
Rimal, D.
Adikaram, D.
Raue, B. A.
Weinstein, L. B.
Arrington, J.
Brooks, W. K.
Ungaro, M.
Adhikari, K. P.
Afanasev, A. V.
Akbar, Z.
Pereira, S. Anefalos
Badui, R. A.
Ball, J.
Baltzell, N. A.
Battaglieri, M.
Batourine, V.
Bedlinskiy, I.
Biselli, A. S.
Boiarinov, S.
Briscoe, W. J.
Bültmann, S.
Burkert, V. D.
Carman, D. S.
Celentano, A.
Chetry, T.
Ciullo, G.
Clark, L.
Colaneri, L.
Cole, P. L.
Compton, N.
Contalbrigo, M.
Cortes, O.
Crede, V.
D'Angelo, A.
Dashyan, N.
De Vita, R.
Deur, A.
Djalali, C.
Dupre, R.
Egiyan, H.
Fassi, L. El
Eugenio, P.
Fanchini, E.
Fedotov, G.
Fersch, R.
Filippi, A.
Fleming, J. A.
Forest, T. 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.
Ishkhanov, B. S.
Isupov, E. L.
Jenkins, D.
Jiang, H.
Joosten, S.
Keller, D.
Khachatryan, G.
Khandaker, M.
Kim, W.
Klein, A.
Klein, F. J.
Kubarovsky, V.
Kuhn, S. E.
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.
Munevar, E.
Camacho, C. Munoz
Nadel-Turonski, P.
Ni, A.
Abstract
Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron
scattering experiments show a significant disagreement that grows with the squared four-momentum transfer (Q2).
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 R2γ , the TPE correction to the elasticscattering cross section, at Q2 ≈ 0.85 and 1.45 GeV2 over a large range of virtual photon polarization ε.
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 Q2 ≈ 2 GeV2
. These data are consistent with an increase in R2γ with
decreasing ε at Q2 ≈ 0.85 and 1.45 GeV2
. There are indications of a slight increase in R2γ with Q2.
scattering experiments show a significant disagreement that grows with the squared four-momentum transfer (Q2).
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 R2γ , the TPE correction to the elasticscattering cross section, at Q2 ≈ 0.85 and 1.45 GeV2 over a large range of virtual photon polarization ε.
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 Q2 ≈ 2 GeV2
. These data are consistent with an increase in R2γ with
decreasing ε at Q2 ≈ 0.85 and 1.45 GeV2
. There are indications of a slight increase in R2γ with Q2.
Subjects
