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Register a new userStudy of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range
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B. Gou
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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.
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We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasi-elastic scattering on the deuteron, at backward angles (lab scattering angle of 108 degrees) for Q2 = 0.22 GeV^2/c^2 and 0.63 GeV^2/c^2 at beam energies of 362 MeV and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (piN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasi-static deuterium approximation, and is also in agreement with theory.
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 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.
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.
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)$^2$.
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