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Register a new userMeasurement of the Transverse Beam Spin Asymmetry in Elastic Electron Proton Scattering and the Inelastic Contribution to the Imaginary Part of the Two-Photon Exchange Amplitude
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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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We report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, $A^{ep}_{perp}$, at five beam energies from 315.1 MeV to 1508.4 MeV and at a scattering angle of $30^{circ} < theta < 40^{circ}$. The covered $Q^2$ values are 0.032, 0.057, 0.082, 0.218, 0.613 (GeV/c)$^2$. The measurement clearly indicates significant inelastic contributions to the two-photon-exchange (TPE) amplitude in the low-$Q^2$ kinematic region. No theoretical calculation is able to reproduce our result. Comparison with a calculation based on unitarity, which only takes into account elastic and $mathrm{pi N}$ inelastic intermediate states, suggests that there are other inelastic intermediate states such as $mathrm{pi pi N}$, $mathrm{K Lambda}$ and $mathrm{eta N}$. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.
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.
A beam-normal single-spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable related to the imaginary part of the two-photon exchange process. We report a 2% precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering with a mean scattering angle of theta_lab = 7.9 degrees and a mean energy of 1.149 GeV. The asymmetry result is B_n = -5.194 +- 0.067 (stat) +- 0.082 (syst) ppm. This is the most precise measurement of this quantity available to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles (theta_lab -> 0) where they should be most reliable.
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.
The transverse beam spin induced asymmetry is calculated for the scattering of transversally polarized electrons on a proton target within a realistic model. Such asymmetry is due to the interference between the Born amplitude and the imaginary part of two photon exchange amplitude. In particular, the contribution of non-excited hadron state (elastic) to the two photon amplitude is calculated. The elastic contribution requires infrared divergences regularization and can be expressed in terms of numerical integrals of the target form factor. The inelastic channel corresponding to the one pion hadronic state contribution is enhanced by squared logarithmic terms. We show that the ratio of elastic over inelastic channel is of the order of 0.3 and cannot be ignored. Enhancement effects due to the decreasing of form factors bring the transverse beam asymmetry to values as large as $10^{-4}$ for particular kinematical conditions.
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