Session P3: QCD Structure of the Nucleon

New measurement of the EMC effect in light nuclei

Twenty-six years ago, CERN physicists made the unexpected observation that the quark distribution in a nucleus is not just the sum of the quark distributions of its nucleons. This raised the logical question on the possible modification of nucleon structure in the nuclear medium. Describing the behavior of the nucleon in-medium has been one of the challenges of theoretical nuclear physics. Even after years of study, including additional measurements at CERN, SLAC and HERA, there is not a clear consensus. It has been broadly accepted that nuclear binding and Fermi motion should be included in any realistic models. Yet still some ``by-products" of theoretical calculations, i.e. nucleon swelling or pion enhancement, are in contradiction with other experimental results. JLab Hall C experiment E03-103 was designed for a precision measurement of the EMC effect in light nuclei, where reliable calculations can be performed. The experiment collected the first data on the EMC effect in $^3$He at large $x$. It also improved the precision on the EMC ratio of medium to heavy nuclei. Our light nuclei results suggest a local density dependence in which the EMC effect would be sensitive to the detailed structure of the nuclei. In this talk, I will give a short review of the EMC effect with particular emphasis on new insights emerging from the recent JLab results.   

Strange sea contribution to the ground state charge and magnetization of the nucleon

The contributions of strange quarks to nucleon properties have been studied in several observables: to the momentum from deep inelastic neutrino scattering, to the spin with polarized deep inelastic electron scattering and to the mass with pion-nucleon scattering. In order to extract the contribution of strange quarks to the ground state charge and magnetization distributions of the nucleon, several Parity Violating (PV) electron scattering experiments have been carried out. These experiments involve measurement of the helicity dependent cross section of elastically scattered polarized electrons from an unpolarized target. During this talk, I will be focusing on the G0 experiment at the Thomas Jefferson National Accelerator Facility. G0 recently measured the parity violating asymmetry in the cross section for polarized electrons scattered at backward angles off liquid hydrogen and deuterium. Measurements were made at two momentum transfers: 0.23 and 0.62 (GeV/$c$)$^{2}$. Combined with earlier forward angle measurements on a hydrogen target, also from the G0 experiment, the contribution of strange quarks to the proton's charge and magnetization distributions can be determined. These measurements also allow the extraction of the isovector axial form factor as seen in electron scattering. Final results of the complete separation of the strange electric, strange magnetic and the isovector axial form factors at these two kinematic points are presented. A variety of recent theoretical predictions of these form factors are discussed.   

Novel Features of Hadronic Form Factors

The increasingly common use of the double-polarization technique to measure the nucleon electromagnetic form factors, in the last 15 years, has resulted in a dramatic improvement of the quality of all four nucleon electromagnetic form factors, $G_{Ep}$, $G_{Mp}$, $G_{En}$ and $G_{Mn}$. It has also completely changed our understanding of the proton structure, having resulted in a distinctly different $Q^2$-dependence for $G_{Ep}$ and $G_{Mp}$, contradicting the prevailing wisdom of the 1990's based on cross section measurements and the Rosenbluth separation method, namely that $G_{Ep}$ and $G_{Mp}$ obey a ``scaling'' relation $\mu G_{Ep}\sim G_{Mp}$. A direct consequence of the faster decrease of $G_{Ep}$ revealed by the JLab polarization experiments was the disappearance of the early scaling $F_2/F_1 \sim 1/Q^2$ predicted by perturbative QCD. Electromagnetic form factors encode the information on the structure of a strongly interacting many-body system of quarks and gluons, such as the nucleon. Much theoretical efforts have been made to understand the nucleon form factors. This reflects the fact that a direct calculation of nucleon form factors from the underlying theory, Quantum Chromodynamics, is complicated as it requires, in the few GeV momentum transfer region, non-perturbative methods. Therefore, in practice it involves approximations which often have a limited range of applicability. The unexpected results of the nucleon electromagnetic form factors using double-polarization high-precision experiments, have challenged our theoretical understanding of the structure of the nucleon. They have triggered several new theoretical developments, which will be discussed.