Nucleon electromagnetic structure: past, present, and future

We present the experimental status of electromagnetic hadron form factors. New and surprising results, based on polarization measurements, have been recently obtained for the electric proton and neutron form factors. In particular, the electric and magnetic distributions inside the proton appear not to be the same, in disagreement with results extracted from the unpolarized cross section, using the Rosenbluth separation. The new findings have given rise to a large number of papers and different speculations, as they question directly the models of nucleon structure and the reaction mechanism itself (based on $1\gamma$-exchange), with a possible revision of the calculation of radiative corrections, two-photon contribution etc. New data in time-like region are also available, through annihilation reactions. A large interest in this field arises, due also to the possibility of new measurements in polarized electron nucleon elastic scattering at JLab, and also in the time-like region, at Frascati and at the future FAIR international facility.Comment: 10 pages, 3 figures, to be included in a special issue of Nuovo Cimento

Charge asymmetry and symmetry properties

Applying general symmetry properties of electromagnetic interaction, information from electron proton elastic scattering data can be related to charge asymmetry in the annihilation channels $e^++e^-\leftrightarrow \bar p + p$ and to the ratio of the cross section of elastic electron and positron scattering on the proton. A compared analysis of the existing data allows to draw conclusions on the reaction mechanism.Comment: Contribution to XIII Workshop On High Energy Spin Physics (DSPIN), Dubna, Russia September 1 - 5, 2009 8 pages 3 figure

Periodic interference structures in the time-like proton form factor

An intriguing and elusive feature of the timelike hadron form factor is the possible presence of an imaginary part associated to rescattering processes. We find evidence of that in the recent and precise data on the proton timelike form factor measured by the BABAR collaboration. By plotting these data as a function of the 3-momentum of the relative motion of the final proton and antiproton, a systematic sinusoidal modulation is highlighted in the near-threshold region. Our analysis attributes this pattern to rescattering processes at a relative distance of 0.7-1.5 fm between the centers of the forming hadrons. This distance implies a large fraction of inelastic processes in $\bar{p}p$ interactions, and a large imaginary part in the related $e^+e^- \rightarrow \bar{p}p$ reaction because of unitarity.Comment: 5 pages 3 figures - Discussion modified. To appear in Phys Rev Letter

Recent results on hadron form factors

We discuss the recent data on the electric proton form factor, obtained at JLab, which show a spectacular deviation from the commonly assumed dipole behavior. We discuss the implication of these results on the deuteron structure and on the neutron electric form factor: at relatively large $Q^2$ a revision of the deuteron models may be required, and the neutron electric form factor might become even larger than the proton electric form factor.Comment: 17 pages 4 figures Contribution to the Int. Workshop on Relativistic Nuclear Physics from hundreds of MeV to TeV, Varna, Bulgaria, 10-16 September 2001. Replacement of Fig. 1 and relative tex

Polarization phenomena for meson production in nucleon-nucleon collisions

We analyze polarization phenomena for pseudoscalar and vector mesons production in nucleon-nucleon collisions. We identify three energy regions corresponding to different physics and different approaches in the analysis of polarization effects. In the threshold region, characterized by the S-wave production for all final particles, the general symmetry properties of strong interaction can be applied. The region of intermediate energies, T=2-4 GeV, is characterized by the essential role of central i.e. non-peripheral collisions, where only a small number of s-channel states with definite quantum numbers, ${\cal J}^P=1^-$ and $2^+$, contribute. At higher energies, T $\ge$10 GeV, the leading mechanism is the diffractive dissociation and it is especially interesting for baryon spectroscopy. The transition to this region is an open field for experimental research at the Nuclotron.Comment: 25 pages, 9 figures Int. Workshop on Relativistic Nuclear Physics from Hundreds of MeV to TeV, Varna, Bulgaria, 10-16 September 200