Search for Effects Beyond the Born Approximation in Polarization Transfer Observables in $\overrightarrow{e}p$ Elastic Scattering

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 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 $H(\overrightarrow{e},e^{\prime }\overrightarrow{p})$ reaction for three different beam energies at a $Q^2=2.5{\mathrm{GeV}}^2$, spanning a wide range of the kinematic parameter $ϵ$. The ratio $R$, which equals ${\mu }_p{G}_E/G_M$ in the Born approximation, is found to be independent of $ϵ$ at the 1.5% level. The $ϵ$ dependence of the longitudinal polarization transfer component $P_{\ell }$ shows an enhancement of $(2.3\pm 0.6)\%$ relative to the Born approximation at large $ϵ$.

Figure 1

$R$ (left column) and $A_y{P}_{\ell }$ (right column) versus the dispersive $dx/dz$ (vertical) and nondispersive $dy/dz$ (horizontal) slopes for the low-energy setting using the COSY model.

Figure 2

(a) $R$ as a function of $ϵ$ with statistical uncertainties, filled circles from this experiment and open triangle from [7]. The theoretical predictions are from [14] (hadronic), [13] (GPD), [19] (COZ and BLW), and [17] (SF) offset for clarity by $-0.006$ with respect to the fit. The one-parameter fit result is $R=0.6923\pm 0.0058$. (b) $P_{\ell }/P_{\ell }^{\mathrm{Born}}$ as a function of $ϵ$. The point-to-point systematic uncertainties, shown with a band in both panels, are relative to the largest $ϵ$ kinematic in (a) and relative to the smallest $ϵ$ kinematic in  (b). The star indicates the $ϵ$ value at which the analyzing power is determined.