Phys.Rev.Lett.106:132501,2011 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 $\epsilon$.
Double-spin asymmetries and absolute cross sections were measured at large Bjorken \(x\) (0.25 \( \le x \le \) 0.90), in both the deep-inelastic and resonance regions, by scattering longitudinally ...polarized electrons at beam energies of 4.7 and 5.9 GeV from a transversely and longitudinally polarized \(^3\)He target. In this dedicated experiment, the spin structure function \(g_2\) on \(^3\)He was determined with precision at large \(x\), and the neutron twist-three matrix element \(d_2^n\) was measured at \(\left< Q^2\right>\) of 3.21 and 4.32 GeV\(^2\)/\(c^2\), with an absolute precision of about \(10^{-5}\). Our results are found to be in agreement with lattice QCD calculations and resolve the disagreement found with previous data at \(\left< Q^2\right> =\) 5 GeV\(^2\)/\(c^2\). Combining \(d_2^n\) and a newly extracted twist-four matrix element, \(f_2^n\), the average neutron color electric and magnetic forces were extracted and found to be of opposite sign and about 30 MeV/fm in magnitude.
Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon's ...quark constituents; indeed, recent proton data have attracted intense theoretical interest. In this letter, we report new measurements of the proton electromagnetic form factor ratio using the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5 GeV2. By extending the range of Q2 for which GEp is accurately determined by more than 50%, these measurements will provide significant constraints on models of nucleon structure in the non-perturbative regime.
We present new data for the polarization observables of the final state proton in the \(^{1}H(\vec{\gamma},\vec{p})\pi^{0}\) reaction. These data can be used to test predictions based on hadron ...helicity conservation (HHC) and perturbative QCD (pQCD). These data have both small statistical and systematic uncertainties, and were obtained with beam energies between 1.8 and 5.6 GeV and for \(\pi^{0}\) scattering angles larger than 75\(^{\circ}\) in center-of-mass (c.m.) frame. The data extend the polarization measurements data base for neutral pion photoproduction up to \(E_{\gamma}=5.6 GeV\). The results show non-zero induced polarization above the resonance region. The polarization transfer components vary rapidly with the photon energy and \(\pi^{0}\) scattering angle in c.m. frame. This indicates that HHC does not hold and that the pQCD limit is still not reached in the energy regime of this experiment.
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 \(\epsilon\).
