Narrow Results

Nucleon electromagnetic form factors are fundamental quantities related to the charge and magnetization distributions inside the nucleon. Understanding the nucleon electromagnetic structure in terms of the underlying quark and gluon degrees of freedom of quantum chromodynamics is a challenging and urgent task. The nucleon electromagnetic form factors have been studied in the past extensively in unpolarized electron scattering experiments. With the development in polarized beam, recoil polarimetry, and polarized target technologies, polarization experiments have provided more precise data on these quantities. At the same time, significant theoretical progress in areas ranging from effective field theories to lattice QCD calculations, has been made in describing these data. In this article, we review recent experimental and theoretical progress on this subject. We also provide the future outlook on this topic.

The helicity-dependent coherent π⁰-photoproduction in the reaction γd → π⁰d near the η-threshold is investigated. The calculations are performed within an approach which includes the reaction amplitudes of the impulse approximation (IA), two-step process with intermediate πN- and ηN-rescattering, and the higher order terms in the multiple scattering series for the intermediate ηNN interaction. The contribution of γd → π⁰d to the deuteron spin asymmetry is calculated and its contribution to the Gerasimov–Drell–Hearn (GDH) integral is explicitly evaluated by integration up to a photon energy of 900 MeV. In addition, the helicity E-asymmetry is calculated. The results revealed that the doubly polarized differential cross-sections and the helicity E-asymmetry are sensitive to the interference of rescattering effects, specially at photon energies 600–800 MeV and extreme backward pion angles. The sensitivity of the obtained results for the GDH integral to the choice of NN potential model governing the deuteron wave function is discussed. We find that the deviation among results obtained for the deuteron GDH integral using different deuteron wave functions is quite large.

Tensor target spin asymmetries $T_{2M}$ ($M$=0,1,2) in the reaction $\gamma d\to {\pi }^{-}pp$ are studied for photon energies from $\pi$-threshold up to 1.5GeV with inclusion of rescattering effects. It is shown that the influence of rescattering effects on the tensor target spin asymmetries is sizable in the energy region near $\pi$-threshold. At higher energies, much smaller influence of rescattering effects is seen. The sensitivity of the obtained results to the elementary pion photoproduction operator is investigated and a considerable dependence is found, in particular at forward pion angles. In addition, a comparison with results of other theoretical models is also given. The extracted spin asymmetries are compared with available experimental data and a qualitative agreement is obtained. The predictions presented here may be useful to interpret the recent measurements from the VEPP-3 electron storage ring.

Incoherent ${\pi }^{+}$-photoproduction on the deuteron is investigated for photon energies near threshold with particular focus on beam-target double spin asymmetries. The analysis is based on a $\gamma d\to {\pi }^{+}nn$ reaction model in which realistic elementary amplitudes for $\gamma N\to \pi N$, $NN\to NN$ and $\pi N\to \pi N$ are incorporated. Numerical results on all possible beam-target double spin asymmetries of the differential and total cross-sections in the photon energy region near threshold are presented. Effects of $NN$ and $\pi N$ final-state interactions are investigated and their roles are found to be important.

The 15${}^{\mathrm{\text{th}}}$ International Workshop on Polarized Source Targets and Polarimeters was held at the University of Virginia, Charlottesville during September 9–13, 2013. It was sponsored jointly by the University of Virginia, Jefferson Laboratory and the International Spin Physics Committee. A summary of the workshop was presented.

Dynamically nuclear polarized solid ammonia offers an attractive combination of high polarization, comparatively high dilution factor and high radiation damage resistance as a target material in electron scattering experiments. Polarized ¹⁴NH₃, provided by the University of Virginia Polarized Target Group, was used as target material in two simultaneous experiments at Thomas Jefferson National Accelerator Facility in the spring of 2009. Target polarization performance for both experiments is discussed, as is previously unseen behavior in irradiated ¹⁴NH₃.

The high physics potential of experiments with stored high-energy polarized antiprotons led to the proposal of PAX (Polarized Antiproton eXperiment) for the High Energy Storage Ring (HESR) of the new FAIR facility at GSI (Darmstadt/Germany). It is proposed to polarize a stored antiproton beam by means of spin-filtering with a polarized hydrogen (deuterium) gas target. The feasibility of spin-filtering with protons has been demonstrated in the FILTEX experiment. In an additional depolarization experiment at COSY no influence of electron scattering on the proton polarization was found. Several experimental studies with protons (at COSY/Jülich) as well as antiprotons (at AD/CERN) will be carried out to measure spin-dependent and cross sections. A Polarized Internal gas Target (PIT) surrounded by silicon detectors and immersed into a low-β section has to be set up.