Measurement 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

Abstract

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${}_{\mathrm{stat}}$ $\pm$ \combinedsyspolerrorlowalucor${}_{\mathrm{sys}}$) $\times$ 10${}^{-6}$ and A${}_{\perp }$(Q${}^2$ = \qsquaredaveragedhighII (GeV/c)${}^2$) = (\experimentalasymme tryaluhighcorr $\pm$ \statisticalerrorhigh${}_{\mathrm{stat}}$ $\pm$ \combinedsyspolerrorhighalucor${}_{\mathrm{sys}}$) $\times$ 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.

arXiv comment4 figures, submitted to PRL on Oct. 8

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