398 research outputs found
Non local lagrangians(I): the pion
We define a family of non local and chirally symmetric low energy lagrangians
motivated by theoretical studies on Quantum Chromodynamics. These models lead
to quark propagators with non trivial momentum dependencies. We define the
formalism for two body bound states and apply it to the pion. We study the
coupling of the photon and W bosons with special attention to the
implementation of local gauge invariance. We calculate the pion decay constant
recovering the Goldberger-Treiman and the Gell-Mann-Oakes-Renner relations. We
recover a form of the axial current consistent with PCAC. Finally we study the
pion form factor and we construct the operators involved in its parton
distribution.Comment: Latex, 25 pages with 15 figures; added references for introduction
and section
On BLM scale fixing in exclusive processes
We discuss the BLM scale fixing procedure in exclusive electroproduction
processes in the Bjorken regime. We show that in the case of vector meson
production the usual way to aplly the BLM method fails due to singularities
present in equations fixing the BLM scale. We argue that the BLM scale should
be extracted from the squared amplitudes which are directly related to
observables.Comment: accepted for the publication in Eur.Phys.J.
Dispersion relations and subtractions in hard exclusive processes
We study analytical properties of the hard exclusive processes amplitudes. We
found that QCD factorization for deeply virtual Compton scattering and hard
exclusive vector meson production results in the subtracted dispersion relation
with the subtraction constant determined by the Polyakov-Weiss -term. The
relation of this constant to the fixed pole contribution found by Brodsky,
Close and Gunion and defined by parton distributions is proved, while its
manifestation is spoiled by the small divergence. The continuation to the
real photons limit is considered and the numerical correspondence between
lattice simulations of -term and low energy Thomson amplitude is found.Comment: 4 pages, journal versio
Hard Electroproduction of Hybrid Mesons
We estimate the sizeable cross section for deep exclusive electroproduction
of an exotic hybrid meson in the Bjorken regime. The production
amplitude scales like the one for usual meson electroproduction, {\it i.e.} as
. This is due to the non-vanishing leading twist distribution amplitude
for the hybrid meson, which may be normalized thanks to its relation to the
energy momentum tensor and to the QCD sum rules technique. The hard amplitude
is considered up to next--to--leading order in and we explore the
consequences of fixing the renormalization scale ambiguity through the BLM
procedure.Comment: e-version for "Advanced Studies Institute Conference on Symmetries
and Spin (SPIN-Praha-2004)", Prague, Czech Republic, 5-10 Jul 200
Extraction of the Compton Form Factor H from DVCS measurements at Jefferson Lab
In the framework of Generalised Parton Distributions, we study the
helicity-dependent and independent cross sections measured in Hall A and the
beam spin asymmetries measured in Hall B at Jefferson Laboratory. We perform a
global fit of these data and fits on each kinematical bin. We extract the real
and imaginary parts of the Compton Form Factor under the main
hypothesis of dominance of the Generalised Parton Distribution and twist 2
accuracy. We discuss our results and compare to previous extractions as well as
to the VGG model. We pay extra attention to the estimation of errors on the
extraction of .Comment: 24 pages, 8 figure
Recent and future experimental evidences for exotic mesons in hard reactions
The QCD analysis of the recent experimental data (L3@LEP) of the hard
exclusive production in two photon collisions shows that these data
can be understood as a signal for the existence of an exotic isotensor
resonance with a mass around . We also argue that hard exclusive
reactions are a powerful tool for an experimental study of exotic hybrid mesons
with .Comment: Talk given at XXXIII International Conference on High Energy Physics,
July 26 - August 2, Moscow, Russi
Probing the partonic structure of exotic particles in hard electroproduction
We argue that the electroproduction of exotic particles is a useful tool for
study of their partonic structure. In the case of hybrid mesons, the magnitude
of their cross sections shows that they are accessible for measurements in
existing electroproduction experiments.Comment: 10 pages Latex, 5 figs., talk at Three Days of Hadronic Physics
(HLPR04): Joint Meeting Heidelberg-Liege-Paris-Rostock, Liege, Belgium, 16-18
Dec 200
Hard production of exotic hybrid mesons
Exotic hybrid mesons H, with quantum numbers J^{PC}= 1^{- +} may be copiously
produced in the hard exclusive processes gamma^*(Q^2) gamma -> H and
gamma^*(Q^2) P(p) -> H P(p') because they have a leading twist distribution
amplitude with a sizable coupling constant f_H, which may be estimated through
QCD sum rules. The reaction rates scale in the same way as the corresponding
rates for usual mesons.Comment: 4 pages Latex, talk at XIth International Conference on Elastic and
Diffractive Scattering Chateau de Blois, France, May 15 - 20, 200
On the description of exclusive processes beyond the leading twist approximation
We describe hard exclusive processes beyond the leading twist approximation
in a framework based on the Taylor expansion of the amplitude around the
dominant light-cone directions. This naturally introduces an appropriate set of
non-perturbative correlators whose number is minimalized after taking into
account QCD equations of motion and the invariance under rotation on the
light-cone.
We examplify this method at the twist 3 level and show that the coordinate
and momentum space descriptions are fully equivalent.Comment: 6 pages, 2 figures, modified and slightly expanded version to be
published in Phys.Lett.
QCD factorization of exclusive processes beyond leading twist: gamma*T -> rhoT impact factor with twist three accuracy
We describe a consistent approach to factorization of scattering amplitudes
for exclusive processes beyond the leading twist approximation. The method
involves the Taylor expansion of the scattering amplitude in the momentum space
around the dominant light-cone direction and thus naturally introduces an
appropriate set of non-perturbative correlators which encode effects not only
of the lowest but also of the higher Fock states of the produced particle. The
reduction of original set of correlators to a set of independent ones is
achieved with the help of equations of motion and invariance of the scattering
amplitude under rotation on the light-cone. We compare the proposed method with
the covariant method formulated in the coordinate space, based on the operator
product expansion. We prove the equivalence of two proposed parametrizations of
the rhoT distribution amplitudes. As a concrete application, we compute the
expressions of the impact factor for the transition of virtual photon to
transversally polarised rho-meson up to the twist 3 accuracy within these two
quite differents methods and show that they are identical.Comment: 53 pages, 16 figure
- …