21,249 research outputs found
Pion Form Factor in the Light-Front
The pion electromagnetic form factor is calculated with a light-front quark
model. The "plus" and "minus" component of the electromagnetic current are used
to calculate the electromagnetic form factor in the Breit frame with two models
for the q\bar{q} vertex. The light front constituent quark models describes
very well hadronic wave function for pseudo-scalar and vector particles.
Symmetry problems arinsing in the light-front approach are solved by the pole
dislocation method. The results are compared with new experimental data and
with other quark models.Comment: 4 pages,1 figure (eps), Latex,AIP style.To appear in the proceedings
"IX Hadron Physics and VII Relativistic Aspects of Nuclear Physics: A Joint
Meeting on QCD and QGP, Hadron Physics-RANP,2004,Angra dos Reis, Rio de
Janeiro,Brazil.(some references are added and small mistakes are corrected.
Spin-1 Particle in the Light-Front Approach
The electromagnetic current of spin-1 composite particles does not transform
properly under rotations if only the valence contribution is considered in the
light-front model. In particular, the plus component of the current, evaluated
only for the valence component of the wave function, in the Drell-Yan frame
violates rotational symmetry, which does not allow a unique calculation of the
electromagnetic form-factors. The prescription suggested by Grach and
Kondratyuk [Sov. J. Nucl. Phys. 38, 198 (1984)] to extract the form factors
from the plus component of the current, eliminates contributions from pair
diagrams or zero modes, which if not evaluated properly cause the violation of
the rotational symmetry. We address this problem in an analytical and covariant
model of a spin-1 composite particle.Comment: To appear Brazilian Journal of Physics (2004), 4 pages, no figures.
Use multicols.st
A combined study of the pion's static properties and form factors
We study consistently the pion's static observables and the elastic and
\gamma*\gamma -> \pi^0 transition form factors within a light-front model.
Consistency requires that all calculations are performed within a given model
with the same and single adjusted length or mass-scale parameter of the
associated pion bound-state wave function. Our results agree well with all
extent data including recent Belle data on the \gamma*\gamma -> \pi^0 form
factor at large q^2, yet the BaBar data on this transition form factor resists
a sensible comparison. We relax the initial constraint on the bound-state wave
function and show the BaBar data can partially be accommodated. This, however,
comes at the cost of a hard elastic form factor not in agreement with
experiment. Moreover, the pion charge radius is about 40% smaller than its
experimentally determined value. It is argued that a decreasing charge radius
produces an ever harder form factor with a bound-state amplitude difficultly
reconcilable with soft QCD. We also discuss why vector dominance type models
for the photon-quark vertex, based on analyticity and crossing symmetry, are
unlikely to reproduce the litigious transition form factor data.Comment: 14 pages, 7 figures, 2 Tables; minor changes; 2 new references added
(Ref. 54 and 57); version to be published in Few Body Physic
Light-Front projection of spin-1 electromagnetic current and zero-modes
The issue of the contribution of zero-modes to the light-front projection of
the electromagnetic current of phenomenological models of vector particles
vertices is addressed in the Drell-Yan frame. Our analytical model of the
Bethe-Salpeter amplitude of a spin-1 fermion-antifermion composite state gives
a physically motivated light-front wave function symmetric by the exchange of
the fermion and antifermion, as in the -meson case. We found that among
the four independent matrix elements of the plus component in the light-front
helicity basis only the one carries zero mode contributions. Our
derivation generalizes to symmetric models, important for applications, the
above conclusion found for a simplified non-symmetrical form of the spin-1
Bethe-Salpeter amplitude with photon-fermion point-like coupling and also for a
smeared fermion-photon vertex model.Comment: Use elservier style. 14 page
Variational Formulation for Quaternionic Quantum Mechanics
A quaternionic version of Quantum Mechanics is constructed using the
Schwinger's formulation based on measurements and a Variational Principle.
Commutation relations and evolution equations are provided, and the results are
compared with other formulations.Comment: Talk given at ICCA*, May 26-30 of 2008, Campinas, SP, Brazil. 18
pages, no figur
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