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17 April 2024

» arxiv » nucl-ex/0410013

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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
F. E. Maas ; K. Aulenbacher ; S. Baunack ; L. Capozza ; J. Diefenbach ; B. Gläser ; Y. Imai ; T. Hammel ; D. von Harrach ; E.-M. Kabub ; R. Kothe ; J. H. Lee ; A. Sanchez-Lorente ; E. Schilling ; D. Schwaab ; G. Stephan ; G. Weber ; C. Weinrich ; I. Altarev ; J. Arvieux ; M. Elyakoubi ; R. Frascaria ; R. Kunne ; M. Morlet ; S. Ong ; J. Vandewiele ; S. Kowalski ; R. Suleiman ; S. Taylor ;
Date:	9 Oct 2004
Journal:	Phys.Rev.Lett. 94 (2005) 082001
Subject:	nucl-ex
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 $30^circ < heta_e < 40^circ$. The measured transverse asymmetries are A$_{perp}$(Q$^2$ = qsquaredaveragedlow (GeV/c)$^2$) = (experimentalasymmetry alulowcorr $pm$ statisticalerrorlow$_{ m stat}$ $pm$ combinedsyspolerrorlowalucor$_{ m sys}$) $ imes$ 10$^{-6}$ and A$_{perp}$(Q$^2$ = qsquaredaveragedhighII (GeV/c)$^2$) = (experimentalasymme tryaluhighcorr $pm$ statisticalerrorhigh$_{ m stat}$ $pm$ combinedsyspolerrorhighalucor$_{ m sys}$) $ imes$ 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.
Source:	arXiv, nucl-ex/0410013
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