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.
Precise measurements of the proton electromagnetic form factor ratio $R = mu_p G_E^p/G_M^p$ using the polarization transfer method at Jefferson Lab have revolutionized the understanding of nucleon structure by revealing the strong decrease of $R$ wit
h momentum transfer $Q^2$ for $Q^2 gtrsim 1$ GeV$^2$, in strong disagreement with previous extractions of $R$ from cross section measurements. In particular, the polarization transfer results have exposed the limits of applicability of the one-photon-exchange approximation and highlighted the role of quark orbital angular momentum in the nucleon structure. The GEp-II experiment in Jefferson Labs Hall A measured $R$ at four $Q^2$ values in the range 3.5 GeV$^2 le Q^2 le 5.6$ GeV$^2$. A possible discrepancy between the originally published GEp-II results and more recent measurements at higher $Q^2$ motivated a new analysis of the GEp-II data. This article presents the final results of the GEp-II experiment, including details of the new analysis, an expanded description of the apparatus and an overview of theoretical progress since the original publication. The key result of the final analysis is a systematic increase in the results for $R$, improving the consistency of the polarization transfer data in the high-$Q^2$ region. This increase is the result of an improved selection of elastic events which largely removes the systematic effect of the inelastic contamination, underestimated by the original analysis.
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 tr
ansfer 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 $H(vec{e},evec{p})$ 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 $epsilon$.
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 nucleons quark constituents; indeed, recent proton data have a
ttracted 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.
