492 research outputs found
Interpretation of the Flavor Dependence of Nucleon Form Factors in a Generalized Parton Distribution Model
We give an interpretation of the and quarks contributions to the
nucleon electromagnetic form factors for values of the four-momentum transfer
in the multi-GeV region where flavor separated data have been recently made
available. The data show, in particular, a suppression of quarks with
respect to quarks at large momentum transfer. %and constant ratios of the
flavor dependent Pauli to Dirac form factors ratios. This trend can be
explained using a reggeized diquark model calculation of generalized parton
distributions, thus providing a correlation between momentum and coordinate
spaces, both of which are necessary in order to interpret the partonic
substructure of the form factors. We extend our discussion to the second
moments of generalized parton distributions which are believed to contribute to
partonic angular momentum.Comment: 22 pages, 19 figures; results and figures added and changed, tables
added, formulae added, major rewriting of tex
Explicit Model Realizing Parton-Hadron Duality
We present a model that realizes both resonance-Regge (Veneziano) and
parton-hadron (Bloom-Gilman) duality. We first review the features of the
Veneziano model and we discuss how parton-hadron duality appears in the
Bloom-Gilman model. Then we review limitations of the Veneziano model, namely
that the zero-width resonances in the Veneziano model violate unitarity and
Mandelstam analyticity. We discuss how such problems are alleviated in models
that construct dual amplitudes with Mandelstam analyticity (so-called DAMA
models). We then introduce a modified DAMA model, and we discuss its
properties. We present a pedagogical model for dual amplitudes and we construct
the nucleon structure function F2(x,Q2). We explicitly show that the resulting
structure function realizes both Veneziano and Bloom-Gilman duality.Comment: 11 pages, 8 figure
Consistent Analysis of O(\alpha_s) Corrections to Pion Elastic Form Factor
We examine the role of O(\alpha_s) perturbative corrections to the pion
elastic form factor F_\pi(Q^2). We express the quark current three-point
function in terms of light cone variables and use Borel transformation to
simultaneously model the Feynman mechanism contribution determined by the soft
part of the pion light cone wave function and the hard term involving one gluon
exchange. We find that for Q^2 \sim 5 GeV^2 the total one gluon exchange
contribution may reach 40% of the soft contribution, even though its hard,
factorization scale dependent part remains relatively small.Comment: 15 pages; version to appear in Phys. Rev.
Determination of the pole position of the lightest hybrid meson candidate
Mapping states with explicit gluonic degrees of freedom in the light sector
is a challenge, and has led to controversies in the past. In particular, the
experiments have reported two different hybrid candidates with spin-exotic
signature, pi1(1400) and pi1(1600), which couple separately to eta pi and eta'
pi. This picture is not compatible with recent Lattice QCD estimates for hybrid
states, nor with most phenomenological models. We consider the recent partial
wave analysis of the eta(') pi system by the COMPASS collaboration. We fit the
extracted intensities and phases with a coupled-channel amplitude that enforces
the unitarity and analyticity of the S-matrix. We provide a robust extraction
of a single exotic pi1 resonant pole, with mass and width 1564 +- 24 +- 86 MeV
and 492 +- 54 +- 102 MeV, which couples to both eta(') pi channels. We find no
evidence for a second exotic state. We also provide the resonance parameters of
the a2(1320) and a2'(1700).Comment: 6 pages + 3 pages of supplemental material. Version to appear on
Phys.Rev.Let
Boost operators in Coulomb-gauge QCD: the pion form factor and Fock expansions in phi radiative decays
In this article we rederive the Boost operators in Coulomb-Gauge Yang-Mills
theory employing the path-integral formalism and write down the complete
operators for QCD. We immediately apply them to note that what are usually
called the pion square, quartic... charge radii, defined from derivatives of
the pion form factor at zero squared momentum transfer, are completely blurred
out by relativistic and interaction corrections, so that it is not clear at all
how to interpret these quantities in terms of the pion charge distribution. The
form factor therefore measures matrix elements of powers of the QCD boost and
Moeller operators, weighted by the charge density in the target's rest frame.
In addition we remark that the decomposition of the eta' wavefunction in
quarkonium, gluonium, ... components attempted by the KLOE collaboration
combining data from phi radiative decays, requires corrections due to the
velocity of the final state meson recoiling against a photon. This will be
especially important if such decompositions are to be attempted with data from
J/psi decays.Comment: 14 pages, 4 figure
Towards a Relativistic Description of Exotic Meson Decays
This work analyses hadronic decays of exotic mesons, with a focus on the
lightest one, the , in a fully relativistic formalism,
and makes comparisons with non-relativistic results. We also discuss Coulomb
gauge decays of normal mesons that proceed through their hybrid components. The
relativistic spin wave functions of mesons and hybrids are constructed based on
unitary representations of the Lorentz group. The radial wave functions are
obtained from phenomenological considerations of the mass operator. Fully
relativistic results (with Wigner rotations) differ significantly from
non-relativistic ones. We also find that the decay channels are favored, in agreement with results obtained using
other models.Comment: 14 pages, 7 figure
Chiral Extrapolation, Renormalization, and the Viability of the Quark Model
The relationship of the quark model to the known chiral properties of QCD is
a longstanding problem in the interpretation of low energy QCD. In particular,
how can the pion be viewed as both a collective Goldstone boson quasiparticle
and as a valence quark antiquark bound state where universal hyperfine
interactions govern spin splittings in the same way as in the heavy quark
systems. We address this issue in a simplified model which; however, reproduces
all features of QCD relevant to this problem. A comparison of the many-body
solution to our model and the constituent quark model demonstrates that the
quark model is sufficiently flexible to describe meson hyperfine splitting
provided proper renormalization conditions and correct degrees of freedom are
employed consistently.Comment: 6 pages, 2 eps figures, uses revtex. Version to appear in Phys. Rev.
Let
On the infrared behaviour of Gluons and Ghosts in Ghost-Antighost symmetric gauges
To investigate the possibility of a ghost-antighost condensate the coupled
Dyson--Schwinger equations for the gluon and ghost propagators in Yang--Mills
theories are derived in general covariant gauges, including ghost-antighost
symmetric gauges. The infrared behaviour of these two-point functions is
studied in a bare-vertex truncation scheme which has proven to be successful in
Landau gauge. In all linear covariant gauges the same infrared behaviour as in
Landau gauge is found: The gluon propagator is infrared suppressed whereas the
ghost propagator is infrared enhanced. This infrared singular behaviour
provides indication against a ghost-antighost condensate. In the
ghost-antighost symmetric gauges we find that the infrared behaviour of the
gluon and ghost propagators cannot be determined when replacing all dressed
vertices by bare ones. The question of a BRST invariant dimension two
condensate remains to be further studied.Comment: 34 pages, 6 figures, Version to be published in Phys. Rev.
Propagators in Coulomb gauge from SU(2) lattice gauge theory
A thorough study of 4-dimensional SU(2) Yang-Mills theory in Coulomb gauge is
performed using large scale lattice simulations. The (equal-time) transverse
gluon propagator, the ghost form factor d(p) and the Coulomb potential V_{coul}
(p) ~ d^2(p) f(p)/p^2 are calculated. For large momenta p, the gluon propagator
decreases like 1/p^{1+\eta} with \eta =0.5(1). At low momentum, the propagator
is weakly momentum dependent. The small momentum behavior of the Coulomb
potential is consistent with linear confinement. We find that the inequality
\sigma_{coul} \ge \sigma comes close to be saturated. Finally, we provide
evidence that the ghost form factor d(p) and f(p) acquire IR singularities,
i.e., d(p) \propto 1/\sqrt{p} and f(p) \propto 1/p, respectively. It turns out
that the combination g_0^2 d_0(p) of the bare gauge coupling g_0 and the bare
ghost form factor d_0(p) is finite and therefore renormalization group
invariant.Comment: 10 pages, 7 figure
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