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.