42 research outputs found
JaxoDraw: A graphical user interface for drawing Feynman diagrams
JaxoDraw is a Feynman graph plotting tool written in Java. It has a complete
graphical user interface that allows all actions to be carried out via mouse
click-and-drag operations in a WYSIWYG fashion. Graphs may be exported to
postscript/EPS format and can be saved in XML files to be used in later
sessions. One of the main features of JaxoDraw is the possibility to produce
LaTeX code that may be used to generate graphics output, thus combining the
powers of TeX/LaTeX with those of a modern day drawing program. With JaxoDraw
it becomes possible to draw even complicated Feynman diagrams with just a few
mouse clicks, without the knowledge of any programming language.Comment: 15 pages, no figures; typos corrected; visit the JaxoDraw home page
at http://altair.ific.uv.es/~JaxoDraw/home.htm
Relationship of the 3P0 decay model to other strong decay models
The 3P0 decay model is briefly reviewed. Possible improvements, partly
motivated by the examination of a microscopic description of a quark -
anti-quark pair creation, are considered. They can provide support for the
one-body character of the model which, otherwise, is difficult to justify. To
some extent, they point to a boost effect that most descriptions of processes
involving a pair creation cannot account for.Comment: 4 pages, 2 .eps figures; Contribution to the BARYONS 2002 Conference,
3.-8. March 2002, JLab, US
Comparison of Form Factors Calculated with Different Expressions for the Boost Transformation
The effect of different boost expressions is considered for the calculation
of the ground-state form factor of a two-body system made of scalar particles
interacting via the exchange of a scalar boson. The aim is to provide an
uncertainty range on methods employed in implementing these effects as well as
an insight on their relevance when an ``exact'' calculation is possible. Using
a wave function corresponding to a mass operator that has the appropriate
properties to construct the generators of the Poincar\'{e} algebra in the
framework of relativistic quantum mechanics, form factors are calculated using
the boost transformations pertinent to the instant, front and point forms of
this approach. Moderately and strongly bound systems are considered with masses
of the exchanged boson taken as zero, 0.15 times the constituent mass , and
infinity. In the first and last cases, a comparison with ``exact'' calculations
is made (Wick-Cutkosky model and Feynman triangle diagram). Results with a
Galilean boost are also given. Momentum transfers up to are
considered. Emphasis is put on the contribution of the single-particle current,
as usually done. It is found that the present point-form calculations of form
factors strongly deviate from all the other ones, requiring large contributions
from two-body currents. Different implementations of the point-form approach,
where the role of these two-body currents would be less important, are
sketched.Comment: Version as accepted for publication, added 6 pages of explanatorial
materia
"Point-form" estimate of the pion form factor revisited
The pion form factor calculation in the ``point-form'' of relativistic
quantum mechanics is re-considered. Particular attention is given to the
relation between the momentum of the system and the momentum transfer as well
as to the quark current.Comment: 3 pages, 2 figures, contribution to the 17th International IUAP
Conference on Few-Body Problems in Physics, 5-10 June 2003, Durham (USA
Hadronic Decays of N and \Delta Resonances in a Chiral Quark Model
\pi and \eta decay modes of light baryon resonances are investigated within a
chiral quark model whose hyperfine interaction is based on Goldstone-boson
exchange. For the decay mechanism a modified version of the 3P0 model is
employed. Our primary aim is to provide a further test of the recently proposed
Goldstone-boson-exchange constituent quark model. We compare the predictions
for \pi and \eta decay widths with experiment and also with results from a
traditional one-gluon-exchange constituent quark model. The differences between
nonrelativistic and semirelativistic versions of the constituent quark models
are outlined. We also discuss the sensitivity of the results on the
parametrization of the meson wave function entering the 3P0 model.Comment: 17 pages, 6 eps figure
JaxoDraw: A graphical user interface for drawing Feynman diagrams. Version 2.0 release notes
A new version of the Feynman graph plotting tool JaxoDraw is presented.
Version 2.0 is a fundamental re-write of most of the JaxoDraw core and some
functionalities, in particular importing graphs, are not backward-compatible
with the 1.x branch. The most prominent new features include: drawing of Bezier
curves for all particle modes, on-the-fly update of edited objects, multiple
undo/redo functionality, the addition of a plugin infrastructure, and a general
improved memory performance. A new LaTeX style file is presented that has been
written specifically on top of the original axodraw.sty to meet the needs of
this this new version.Comment: 17 pages, 1 figur
Spectator-model operators in point-form relativistic quantum mechanics
We address the construction of transition operators for electromagnetic,
weak, and hadronic reactions of relativistic few-quark systems along the
spectator model. While the problem is of relevance for all forms of
relativistic quantum mechanics, we specifically adhere to the point form, since
it preserves the spectator character of the corresponding transition operators
in any reference frame. The conditions imposed on the construction of
point-form spectator-model operators are discussed and their implications are
exemplified for mesonic decays of baryon resonances within a relativistic
constituent quark model.Comment: 10 pages, 4 figures, updated version accepted for publication in
Europ. Phys. J.
A note on the QCD evolution of generalized form factors
Generalized form factors of hadrons are objects appearing in moments of the
generalized parton distributions. Their leading-order DGLAP-ERBL QCD evolution
is exceedingly simple and the solution is given in terms of matrix triangular
structures of linear equations where the coefficients are the evolution ratios.
We point out that this solution has a practical importance in analyses where
the generalized form factors are basic objects, e.g., the lattice-gauge studies
or models. It also displays general features of their evolution.Comment: 4 page
Effective boost and "point-form" approach
Triangle Feynman diagrams can be considered as describing form factors of
states bound by a zero-range interaction. These form factors are calculated for
scalar particles and compared to point-form and non-relativistic results. By
examining the expressions of the complete calculation in different frames, we
obtain an effective boost transformation which can be compared to the
relativistic kinematical one underlying the present point-form calculations, as
well as to the Galilean boost. The analytic expressions obtained in this simple
model allow a qualitative check of certain results obtained in similar studies.
In particular, a mismatch is pointed out between recent practical applications
of the point-form approach and the one originally proposed by Dirac.Comment: revised version as accepted for publicatio
Generalized parton distributions of the pion in chiral quark models and their QCD evolution
We evaluate Generalized Parton Distributions of the pion in two chiral quark
models: the Spectral Quark Model and the Nambu-Jona-Lasinio model with a
Pauli-Villars regularization. We proceed by the evaluation of double
distributions through the use of a manifestly covariant calculation based on
the alpha representation of propagators. As a result polynomiality is
incorporated automatically and calculations become simple. In addition,
positivity and normalization constraints, sum rules and soft pion theorems are
fulfilled. We obtain explicit formulas, holding at the low-energy quark-model
scale. The expressions exhibit no factorization in the t-dependence. The QCD
evolution of those parton distributions is carried out to experimentally or
lattice accessible scales. We argue for the need of evolution by comparing the
Parton Distribution Function and the Parton Distribution Amplitude of the pion
to the available experimental and lattice data, and confirm that the
quark-model scale is low, about 320 MeV.Comment: 25 pages, 15 figures, added discussion of the end-point behavio