215 research outputs found
A Model for the Off-forward Structure Functions of the Pion
We extend our model for the pion, which we used previously to calculate its
diagonal structure function, to the off-forward case. The imaginary part of the
off-forward gamma* pi -> gamma* pi scattering amplitude is evaluated in the
chiral limit (m_pi=0) and related to the twist-two and twist-three generalised
parton distributions, H, H3, H3tilde. Non-perturbative effects, linked to the
size of the pion and still preserving gauge invariance, are included.
Remarkable new relations between H, H3 and H3tilde are obtained and discussed.Comment: 9 pages, 20 figures, LaTeX, uses ulg_ptf.cls (included); v2: Form
factors dropped in favour of GPD's, comparison between Wandzura-Wilczek
approximation and our relations adde
A Model for the Generalized Parton Distribution of the Pion
We calculate the off-forward structure function of the pion within a simple
model where the size of the pion is accounted for using a momentum cut-off.
Twist-two and twist-three generalized parton distributions are extracted.
Relations between twist-three and twist-two contributions are obtained, the
origin of which is not kinematical as they differ from those arising from the
Wandzura-Wilczek approximation.Comment: Presented at 6th Conference on Quark Confinement and the Hadron
Spectrum, Villasimius, Sardinia, Italy, 21-25 Sep 2004, 3 pages, 4 figures,
LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included
A toy model for generalised parton distributions
We give the results of a simple model for the diagonal and off-diagonal
valence quark distributions of a pion. We show that structure can be
implemented in a gauge-invariant manner. This explicit model questions the
validityof the momentum sum rule, and gives an explicit counter-example to the
Wandzura-Wilczek ansatz for twist-3 GPD's.Comment: 11 pages, presented at HADRONIC PHYSICS (HLPR 2004): Joint
Meeting-Heidelberg-Liege-Paris-Rostock, Spa, Belgium, 16-18 December 2004, 11
pages, 11 figures, LaTe
The Valence Quark Distribution of the Pion
The pion structure function is investigated in a simple model, where the pion
and its constituent quark fields are coupled through the simplest pseudoscalar
coupling. The imaginary part of the forward gamma* pi -> gamma* pi scattering
amplitude is evaluated and related to the structure functions. It is shown that
the introduction of non-perturbative effects, linked to the size of the pion,
allows a connection with the quark distribution. It is predicted that
higher-twist terms become negligible for Q^2 larger than ~2 GeV^2, that quarks
in the pion have a momentum fraction smaller than in the proton case, and that
the momentum sum rule is violated for the pion.Comment: Presented by J. P. Lansberg at the XXII Physics in Collision
Conference (PIC02), Stanford, California, June 20-22, 2002, 3 pages, 4
figures, LaTeX, uses pic02.sty (included
Pion Structure Function and Violation of the Momentum Sum Rule
We present a method to evaluate the pion structure functions from a box
diagram calculation. Pion and constituent quark fields are coupled through the
simplest pseudoscalar coupling. The gamma^* pi -> q \bar q cross-section is
evaluated and related to the structure functions. We then show that the
introduction of non-perturbative effects, related to the pion size and
preserving gauge invariance, provides us with a straighforward relation with
the quark distribution. It is predicted that higher-twist terms become
negligible for Q^2 larger than about 2 GeV^2 and that quarks in the pion have a
momentum fraction smaller than in the proton. We enlarge the discussion
concerning this violation of the momentum sum rule, emphasizing that the sum
rule is recovered in the chiral limit and also when the finite size condition
is not imposed.Comment: Presented by J.P. Lansberg at the II International Workshop on Hadron
Physics, 25-29 September, 2002, Coimbra, Portugal, 14 pages, 9 figures,
LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included
Confinement and cut-off: a model for the pion quark distribution function
The pion structure function is investigated in a simple pseudo-scalar
coupling model of pion and constituent quark fields. The imaginary part of the
forward Compton scattering amplitude is evaluated. We show that the
introduction of non-perturbative effects, linked through a cut-off to the size
of the pion, allows the reproduction of important features of the pion quark
distribution function.Comment: 3 pages, 1 figur
Generalized Parton Distributions of the Pion
Off-forward structure functions of the pion are investigated in twist-two and
twist-three approximation. A simple model is used for the pion, which allows to
introduce finite size effects, while preserving gauge invariance. Results for
the imaginary parts of the gamma^* pi -> gamma^* pi off-forward amplitude and
of the structure functions are presented. Generalized Callan-Gross relations
are obtained.Comment: 4 pages, 5 figures, LaTeX, uses espcrc2.sty (included), presented at
QCD03 Conference, Montpellier, France, July 200
Microscopic Calculation of in-Medium Proton-Proton Cross Sections
We derive in-medium PROTON-PROTON cross sections in a microscopic model based
upon the Bonn nucleon-nucleon potential and the Dirac-Brueckner approach for
nuclear matter. We demonstrate the difference between proton-proton and
neutron-proton cross sections and point out the need to distinguish carefully
between the two cases. We also find substantial differences between our
in-medium cross sections and phenomenological parametrizations that are
commonly used in heavy-ion reactions.Comment: 9 pages of RevTex and 4 figures (postscript in separate uuencoded
file), UI-NTH-930
- …