67 research outputs found
Dynamical relativistic corrections to the leptonic decay width of heavy quarkonia
We calculate the dynamical relativistic corrections, originating from
radiative one-gluon-exchange, to the leptonic decay width of heavy quarkonia in
the framework of a covariant formulation of Light-Front Dynamics. Comparison
with the non-relativistic calculations of the leptonic decay width of J=1
charmonium and bottomonium S-ground states shows that relativistic corrections
are large. Most importantly, the calculation of these dynamical relativistic
corrections legitimate a perturbative expansion in , even in the
charmonium sector. This is in contrast with the ongoing belief based on
calculations in the non-relativistic limit. Consequences for the ability of
several phenomenological potential to describe these decays are drawn.Comment: 17 pages, 7 figure
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
Complete analysis of spin structure function g1 of He-3
We present a comprehensive analysis of the nuclear effects important in DIS
on polarized He-3 over a wide range of Bjorken x, 10^{-4} \leq x \leq 0.8.
Effects relevant for the extraction of the neutron spin structure function,
g1^n, from the He-3 data are emphasized.Comment: 30 pages, 6 figures, LaTe
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
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
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