Speaker
Description
The Covariant Spectator Quark Model, successful
in the description of the nucleon elastic
form factors revealed by the Jefferson Lab
polarization transfer experiments,
has been extended to the calculation
of transition form factors associated
to several light nucleon resonances $N^\ast$.
The electromagnetic structure of the $N^\ast$
are characterized by the
$\gamma^\ast N \to N^\ast$ transition form factors.
Combining the electromagnetic structure of the
constituent quarks, calibrated by the nucleon data,
with appropriated ansatz for the radial wave functions
of the resonances, we are able to estimate the
transition for factors for the resonances
$N(1440)\frac{1}{2}^+$,
$N(1535)\frac{1}{2}^-$ and
$N(1520)\frac{3}{2}^-$, based on
the parametrization of the nucleon radial wave function
(nucleon shape) with no adjustable parameters.
The calculations take into account exclusively
the effect of the valence quarks
and are in good agreement with the data
for momentum transfer $Q^2 > 2$ GeV$^2$,
with a few exceptions.
The exceptions are discussed in terms of the
role of the valence quarks and the meson cloud.
All the estimates are based on
the parametrization of the nucleon structure,
and can be tested in a near future
for large transfer momentum at the
Jefferson Lab (12 GeV upgrade).
The model can also be extended to the $\Delta(1232)\frac{3}{2}^+$,
with the assistance of the results from lattice QCD, to estimate
the radial wave function of the $\Delta(1232)$ valence quark core.
The model estimates are in agreement with the
empirical data when theoretical and phenomenological
parametrizations of the pion cloud are taken into account.
Summary
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