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Register a new userMeasurement of the two-photon exchange contribution in elastic $ep$ scattering at VEPP-3
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We report on the status of the Novosibirsk experiment on a precision measurement of the ratio $R$ of the elastic $e^+ p$ and $e^- p$ scattering cross sections. Such measurements determine the two-photon exchange effect in elastic electron-proton scattering. The experiment is conducted at the VEPP-3 storage ring using a hydrogen internal gas target. The ratio $R$ is measured with a beam energy of 1.6 GeV (electron/positron scattering angles are $theta = 55 div 75^{circ}$ and $theta = 15 div 25^{circ}$) and 1 GeV ($theta = 65 div 105^{circ}$). We briefly describe the experimental method, paying special attention to the radiative corrections. Some preliminary results are presented.
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The two-photon-exchange (TPE) effect plays a key role to extract the form factors (FFs) of the proton. In this work, we present some exact properties on the TPE effect in the elastic $ep$ scattering based on four types of typical and general interactions. The possible kinematical singularities, the asymptotic behaviors and the branch cuts of the TPE amplitudes are analyzed. The analytic expressions clearly indicate some exact relations between the dispersion relation (DR) method and the hadronic model (HM) method. It suggests that the two methods should be modified to general forms, respectively. After the modifications the new forms give the same results. Furthermore, they automatically and correctly include the contributions due to the seagull interaction, the meson-exchange effect, the contact interactions and the off-shell effect. To analyze the elastic $e^{pm}p$ scattering data sets, the new forms should be used.
We report new precision measurements of the elastic electron-proton scattering cross section for momentum transfer squared (Q$^2$) up to 15.75~gevsq. These data allow for improved extraction of the proton magnetic form factor at high Q$^2$ and nearly double the Q$^2$ range of direct longitudinal/transverse separated cross sections. A comparison of our results to polarization measurements establishes the presence of hard two-photon exchange in the $e$-$p$ elastic scattering cross section at greater than 95% confidence level for Q$^2$ up to 8 (GeV/c)$^2$.
We review recent theoretical and experimental progress on the role of two-photon exchange (TPE) in electron-proton scattering at low to moderate momentum transfers. We make a detailed comparison and analysis of the results of competing experiments on the ratio of e+p to e-p elastic scattering cross sections, and of the theoretical calculations describing them. A summary of the current experimental situation is provided, along with an outlook for future experiments.
Two-photon exchange contributions to elastic electron-proton scattering cross sections are evaluated in a simple hadronic model including the finite size of the proton. The corrections are found to be small in magnitude, but with a strong angular dependence at fixed $Q^2$. This is significant for the Rosenbluth technique for determining the ratio of the electric and magnetic form factors of the proton at high $Q^2$, and partly reconciles the apparent discrepancy with the results of the polarization transfer technique.
We report on a measurement of the asymmetry in the scattering of transversely polarized electrons off unpolarized protons, A$_perp$, at two Q$^2$ values of qsquaredaveragedlow (GeV/c)$^2$ and qsquaredaveragedhighII (GeV/c)$^2$ and a scattering angle of $30^circ < theta_e < 40^circ$. The measured transverse asymmetries are A$_{perp}$(Q$^2$ = qsquaredaveragedlow (GeV/c)$^2$) = (experimentalasymmetry alulowcorr $pm$ statisticalerrorlow$_{rm stat}$ $pm$ combinedsyspolerrorlowalucor$_{rm sys}$) $times$ 10$^{-6}$ and A$_{perp}$(Q$^2$ = qsquaredaveragedhighII (GeV/c)$^2$) = (experimentalasymme tryaluhighcorr $pm$ statisticalerrorhigh$_{rm stat}$ $pm$ combinedsyspolerrorhighalucor$_{rm sys}$) $times$ 10$^{-6}$. The first errors denotes the statistical error and the second the systematic uncertainties. A$_perp$ arises from the imaginary part of the two-photon exchange amplitude and is zero in the one-photon exchange approximation. From comparison with theoretical estimates of A$_perp$ we conclude that $pi$N-intermediate states give a substantial contribution to the imaginary part of the two-photon amplitude. The contribution from the ground state proton to the imaginary part of the two-photon exchange can be neglected. There is no obvious reason why this should be different for the real part of the two-photon amplitude, which enters into the radiative corrections for the Rosenbluth separation measurements of the electric form factor of the proton.
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