exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

Hadron Electrodynamics
IPNO Participation: Egle Tomasi-Gustafsson, Dominique Marchand
Collaboration : IRFU/SPhN (Saclay), JINR-BLTP(Dubna,Russia), NSC-KFTI (Kharkov, Ukraine)
Le but de ces études est la compréhension de la structure interne du nucléon et des mécanismes d’interaction
pour les réactions élastiques, inélastiques et d’annihilation induites par électrons ou hadrons. Nous
avons calculé les observables pour la réaction avec cinématique inverse proton-électron, qui a un intérêt
particulier pour les phénomènes de polarisation. L’étude des corrections radiatives pour la diffusions élastique
électron-proton et pour la diffusion Compton profondément virtuelle, a montré que les corrections
aux ordres supérieurs, deviennent importantes dans le domaine d’énergie couvert par JLab (USA). Les
réactions d ‘annihilation protons-antiprotons (électron-positron) font l’intérêt de programmes d’études auprès
de FAIR (Allemagne) et de BES (Chine), en particulier pour l’étude des facteurs de forme électromagnétiques
du proton dans la région temps. Nous avons suggéré de mesurer le rapport de production de
paires de pions et de kaons qui pourrait mettre en évidence les excitations du vide de QCD.
Proton electron elastic scattering
Electron proton scattering is the most simple reaction
which allows to have information on the hadron
structure. It has been widely studied theoretically
and experimentally, but the present interest is
related to specific kinematical conditions which
could not be reached in the past. The inverse
kinematics p+e→ p+e has to be treated separately,
as approximations, such as neglecting the electron
mass, do not hold anymore. The specificity of this
reaction is that also at a few GeV incident energy
the values of the transferred momenta are very
small. Born approximation can be applied, but at
very small energiesn Coulomb corrections have to
be taken into account. Recent interest is related to
two possible applications: - the possibility to build
beam polarimeters, for high energy polarized proton
beams, in the RHIC energy range, - the possibility
to produce polarized antiprotons beams,
which would open a wide domain of polarization
studies at the FAIR facility. We have calculated the
cross section and the relevant polarization observables
in the Born approximation, as a function of
the scattering angle (or the transferred momentum),
in a wide energy range. Applications to polarimetry
for high energy beams and antiproton
polarization are under evaluation.
High Order Radiative Corrections
The problem of radiative corrections to electron
proton elastic scattering is very important at the
energies and with the experimental precision
achievable in present experiments. In particular,
corrections to the cross section can be very large,
and first order corrections, as usually applied, may
not have the necessary precision. The possibility
that the two photon exchange mechanism (in principle
suppressed by a factor of α, the electromagnetic
fine structure constant) could become important,
has been recently recalled, in connection with
the measurement of proton electromagnetic form
factors in polarized and unpolarized measure-
Fig.1 DVCS helicity asymmetry as a function of
the azimuthal angle: Born (solid line), radiatively
corrected (dashed line) (top); relative value of the
correction in percent (bottom).
ments. A model independent calculation being not
possible, it turns out that theoretical models are
very little constrained and give quantitatively different
results. Our approach is different.
Firstly we calculated the box diagram for ep scattering,
using approximations that are known to
magnify this contribution, which was found of the
order of a percent. Secondly, we calculated exactly
the box diagram due to two photon exchange for
e scattering, which is possible for a pure QED
process. Finally we proved that e scattering constitutes
an upper limit for ep scattering (for the
elastic intermediate state), the reason being that
proton form factors decrease very fast with the momentum
transfer squared and are smaller than the
static values for a structureless particle.
Calculations for high order radiative corrections,
based on the structure functions method were applied
to elastic ep scattering and to Deep Virtual
Compton scattering (DVCS), Fig.1. In both case, it
appeared that high order contributions modify the
size and the dependence of the experimental observables
on the kinematical variables [1].
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