4,217 research outputs found
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 -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 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 interactions, and a large imaginary part in the related 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 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 in Hadronic and Nuclear Processes in Threshold Regime
The threshold region for processes of hadronic and nuclear interactions is
very interesting for a theoretical as well as an experimental point of view. In
this region one can apply different physical methods, starting from classical
current algebra methods - for processes invloving soft pions, through effective
Lagrangian approaches - or perturbative chiral symmetry theory (ChPT). The
essential simplification of the spin structure of matrix elements for threshold
regime results in better understanding of the underlying mechanisms and allows
the analysis of polarization phenomena. A dedicated formalism is developed here
for a wide class of processes including even such complicated processes as
production of pseudoscalar and vector mesons in nucleon-nucleon collisions.
Many interesting physical problems are discussed: hidden strangeness of
nucleons and OZI-violation, -and interactions in S-state,
the determination of P-parity of strange particles. Special attention is
devoted to the analysis of the spin structure and polarization phenomena for
nuclear processes with light nuclei which have important applications in
fundamental astrophysics and in nuclear fusion.Comment: Review article, 83 pages 9 fig
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