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Search for effects beyond the Born approximation in polarization transfer observables in $\vec{e}p$ elastic scattering

Abstract : Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, $G_{E}/G_{M}$, obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange (TPEX) contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic $H(\vec{e},e'\vec{p})$ reaction for three different beam energies at a fixed squared momentum transfer $Q^2 = 2.5$ GeV$^2$, spanning a wide range of the virtual photon polarization parameter, $\epsilon$. From these measured polarization observables, we have obtained separately the ratio $R$, which equals $\mu_p G_{E}/G_{M}$ in the Born approximation, and the longitudinal polarization transfer component $P_\ell$, with statistical and systematic uncertainties of $\Delta R \approx \pm 0.01 \mbox{(stat)} \pm 0.013 \mbox{(syst)}$ and $\Delta P_\ell/P^{Born}_{\ell} \approx \pm 0.006 \mbox{(stat)}\pm 0.01 \mbox{(syst)}$. The ratio $R$ is found to be independent of $\epsilon$ at the 1.5% level, while the $\epsilon$ dependence of $P_\ell$ shows an enhancement of $(2.3 \pm 0.6) %$ relative to the Born approximation at large $\epsilon$.
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Contributor : Sophie Heurteau <>
Submitted on : Friday, April 1, 2011 - 4:47:42 PM
Last modification on : Wednesday, September 16, 2020 - 4:08:31 PM

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M. Meziane, E. J. Brash, R. Gilman, M. K. Jones, W. Luo, et al.. Search for effects beyond the Born approximation in polarization transfer observables in $\vec{e}p$ elastic scattering. Physical Review Letters, American Physical Society, 2011, 106, pp.132501. ⟨10.1103/PhysRevLett.106.132501⟩. ⟨in2p3-00578628⟩

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