124 research outputs found
Quark phase-space distributions and orbital angular momentum
We discuss the Wigner functions of the nucleon which provide
multi-dimensional images of the quark distributions in phase space. They
combine in a single picture all the information contained in the generalized
parton distributions (GPDs) and the transverse-momentum dependent parton
distributions (TMDs). In particular, we present results for the distribution of
unpolarized quarks in a longitudinally polarized nucleon obtained in a
light-cone constituent quark model. We show how quark orbital angular momentum
can be extracted from these distributions and compare it with alternative
definitions given in terms of the GPDs and the TMDs.Comment: 5 pages, 1 figure, 1 table, prepared for the Sixth International
Conference on Quarks and Nuclear Physics, April 16-20, 2012, Ecole
Polytechnique, Palaiseau, Franc
Generalized Transverse-Momentum Dependent Parton Distributions in Light-Cone Quark Models
We discuss the general formalism for the calculation in light-cone quark
models of the fully unintegrated, off-diagonal quark-quark correlator of the
nucleon. The corresponding distributions in impact parameter space are the
Wigner or phase-space distributions. The results obtained in two different
light-cone quark models in the case of unpolarized quarks in an unpolarized
proton are very similar and present a non-trivial shape which can be understood
as due to the orbital motion of the quarks.Comment: Proceedings for the "Light Cone 2010 Relativistic Hadronic and
Particle Physics" workshop, 3 Figs, 8p
Quark Wigner Distributions and Orbital Angular Momentum
We study the Wigner functions of the nucleon which provide multidimensional
images of the quark distributions in phase space. These functions can be
obtained through a Fourier transform in the transverse space of the generalized
transverse-momentum dependent parton distributions. They depend on both the
transverse position and the three-momentum of the quark relative to the
nucleon, and therefore combine in a single picture all the information
contained in the generalized parton distributions and the transverse-momentum
dependent parton distributions. We focus the discussion on the distributions of
unpolarized/longitudinally polarized quark in an unpolarized/longitudinally
polarized nucleon. In this way, we can study the role of the orbital angular
momentum of the quark in shaping the nucleon and its correlations with the
quark and nucleon polarizations. The quark orbital angular momentum is also
calculated from its phase-space average weighted with the Wigner distribution
of unpolarized quarks in a longitudinally polarized nucleon. The corresponding
results obtained within different light-cone quark models are compared with
alternative definitions of the quark orbital angular momentum, as given in
terms of generalized parton distributions and transverse-momentum dependent
parton distributions.Comment: 29 pages, 10 figures, 3 table
The electron in three-dimensional momentum space
We study the electron as a system composed by an electron and a photon, using
lowest order perturbation theory. We derive the leading-twist
transverse-momentum-dependent distribution functions for both the electron and
photon in the dressed electron, thereby offering a three-dimensional
description of the dressed electron in momentum space. To obtain the
distribution functions, we apply both the formalism of light-front wave
function overlap representation and the diagrammatic approach. We perform the
calculations both in light-cone gauge and Feynman gauge, and we present a
detailed discussion of the role of the Wilson lines to obtain gauge-independent
results. We provide numerical results and plots for many of the computed
distributions.Comment: 25 pages, 14 figure
Spatial distribution of angular momentum inside the nucleon
We discuss in detail the spatial distribution of angular momentum inside the
nucleon. We show that the discrepancies between different definitions originate
from terms that integrate to zero. Even though these terms can safely be
dropped at the integrated level, they have to be taken into account at the
density level. Using the scalar diquark model, we illustrate our results and,
for the first time, check explicitly that the equivalence between kinetic and
canonical orbital angular momentum persists at the density level, as expected
in a system without gauge degrees of freedom
The transverse structure of the pion in momentum space inspired by the AdS/QCD correspondence
We study the internal structure of the pion using a model inspired by the
AdS/QCD correspondence. The holographic approach provides the light-front wave
function (LFWF) for the leading Fock state component of the pion. We adopt two
different forms for the LFWF derived from the AdS/QCD soft-wall model, with
free parameters fitted to the available experimental information on the pion
electromagnetic form factor and the leading-twist parton distribution function.
The intrinsic scale of the model is taken as an additional fit parameter.
Within this framework, we provide predictions for the unpolarized transverse
momentum dependent parton distribution (TMD), and discuss its property both at
the scale of the model and after TMD evolution to higher scales that are
relevant for upcoming experimental measurements.Comment: 10 pages, 6 figure
Probing the parton content of the nucleon
The parton content of the nucleon is explored within a meson-cloud model
developed to derive light-cone wave functions for the physical nucleon. The
model is here applied to study electromagnetic form factors, distribution
amplitudes and nucleon-to-meson transition distribution amplitudes.Comment: 10 pages, 6 figures; proceedings of the workshop "Recent Advances in
Perturbative QCD and Hadronic Physics" in Honor of Prof. Anatoly Efremov's
75th Birthday Celebration; to appear in Mod. Phys. Lett.
Reconstructing parton densities at large fractional momenta
Parton distribution functions (PDFs) are nonperturbative objects defined by
nonlocal light-cone correlations. They cannot be computed directly from Quantum
Chromodynamics (QCD). Using a standard lattice QCD approach, it is possible to
compute moments of PDFs, which are matrix elements of local operators.
Recently, an alternative approach has been proposed, based on the introduction
of quasi-parton distribution functions (quasi-PDFs), which are matrix elements
of equal-time spatial correlations and hence calculable on lattice. Quasi-PDFs
approach standard PDFs in the limit of very large longitudinal proton momenta
. This limit is not attainable in lattice simulations, and quasi-PDFs fail
to reproduce PDFs at high fractional longitudinal momenta. In this paper, we
propose a method to improve the reconstruction of PDFs by combining information
from quasi-PDFs and from the Mellin moments of regular PDFs. We test our method
using the diquark spectator model for up and down valence distributions of both
unpolarized and helicity PDFs. In the future, the method can be used to produce
PDFs entirely based on lattice QCD results.Comment: 12 pages, 7 double-panel figures in pdf, RevTeX4-
Beam normal spin asymmetry for the process
We calculate the single spin asymmetry for the
process, for an electron beam polarized normal to the scattering plane. Such
single spin asymmetries vanish in the one-photon exchange approximation, and
are directly proportional to the absorptive part of a two-photon exchange
amplitude. As the intermediate state in such two-photon exchange process is on
its mass shell, the asymmetry allows one to access for the first time the
on-shell as well as electromagnetic
transitions. We present the general formalism to describe the beam normal spin asymmetry, and provide a numerical estimate of its
value using the nucleon, , , and
intermediate states. We compare our results with the first data from the
Qweak@JLab experiment and give predictions for the A4@MAMI experiment.Comment: 18 pages, 9 figure
Hadron tomography through Wigner distributions
We study the Wigner functions of the nucleon which provide multidimensional
images of the quark distributions in phase space and combine in a single
picture all the information contained in the generalized parton distributions
(GPDs) and the transverse-momentum dependent parton distributions (TMDs). In
particular, we present results for the distribution of unpolarized quarks in a
longitudinally polarized nucleon obtained in a light-cone constituent quark
model. Treating the Wigner distribution as it was a classical distribution, we
obtain the quark orbital angular momentum and compare it with alternative
definitions given in terms of the GPDs and the TMDs.Comment: 6 pages, 1 figure, 1 table, Prepared for the Third International
Workshop on Transverse Polarization Phenomena in Hard Scattering
(Transversity2011), Veli Lo\v{s}inj, Croatia, 29 Aug - 2 Sep 201
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