84 research outputs found
Exploring the proton spin structure
Understanding the spin structure of the proton is one of the main challenges
in hadronic physics. While the concepts of spin and orbital angular momentum
are pretty clear in the context of non-relativistic quantum mechanics, the
generalization of these concepts to quantum field theory encounters serious
difficulties. It is however possible to define meaningful decompositions of the
proton spin that are (in principle) measurable. We propose a summary of the
present situation including recent developments and prospects of future
developments.Comment: 8 pages, 1 figure, 2 tables, contribution to the proceedings of the
DAE-BRNS High Energy Physics Symposium 2014, Dec 8-12, Guwahati, Indi
The proton spin decomposition : path dependence and gauge symmetry
We discuss the different types of decomposition of the proton spin. We stress
that, despite their lack of uniqueness, the Chen et al. and Wakamatsu
decompositions are perfectly measurable. We argue that a large part of the
recent controversies boils down to the fact that there actually exist two types
of gauge transformations in the Chen et al. approach, where physical and gauge
degrees of freedom of the gauge potential are explicitly separated. By
carefully distinguishing these two types of gauge transformations, one can
easily understand how the concepts of gauge invariance, Stueckelberg symmetry,
path dependence and measurability are linked to each other.Comment: 9 pages, contribution to the proceedings of the QCD Evolution 2013,
May 6-10, JLab, Newport News, US
Canonical and kinetic decompositions of the proton spin
We propose a short summary of the present situation concerning the proton spin decomposition. We briefly discuss some of the main controversies about the issues of gauge invariance, uniqueness and measurability. As a conclusion, we argue that part of the controversies is actually undecidable
Light-Front Densities for Transversely Polarized Hadrons
We discuss the recent interpretation of quark distribution functions in the
plane transverse to the light-cone direction. Such a mapping is model
independent and allows one to build multidimensional pictures of the hadron and
to develop a semi-classical intuition of the quark dynamics. We comment briefly
the results obtained from the Form Factors of the nucleon. A generalization to
a target with arbitrary spin led to a set of preferred values for the
electromagnetic coupling characterizing structureless particles. Generalized
polarizabilities can also be interpreted in that frame as the distortion of the
charge densities due to an external electromagnetic field. Finally, we present
preliminary results for the Generalized Transverse-Momentum dependent
Distributions which encode in principle the most complete information about
quark distributions.Comment: 8 pages, 2 figures, 1 table; contribution to the proceedings of "The
4th Workshop on Exclusive Reactions at High Momentum Transfer", Jefferson
Lab., May 18-21, 201
Wigner distributions and quark orbital angular momentum
We discuss the quark phase-space or Wigner distributions of the nucleon which
combine in a single picture all the information contained in the generalized
parton distributions and the transverse-momentum dependent parton
distributions. In particular, we present results for the distribution of
unpolarized quarks in a longitudinally polarized nucleon obtained in a
light-front constituent quark model. We show how the quark orbital angular
momentum can be extracted from the Wigner distributions and compare it with
alternative definitions.Comment: 8 pages, 2 figures, 1 table, contribution to the proceedings of the
QCD Evolution workshop 2012, May 14-17, JLa
Transverse pion structure beyond leading twist in constituent models
The understanding of the pion structure as described in terms of
transverse-momentum dependent parton distribution functions (TMDs) is of
importance for the interpretation of currently ongoing Drell-Yan experiments
with pion beams. In this work we discuss the description of pion TMDs beyond
leading twist in a pion model formulated in the light-front constituent
framework. For comparison, we also review and derive new results for pion TMDs
in the bag and spectator models.Comment: 17 pages, 7 figures; v2: modified presentation, updated references;
matches the journal versio
Light-front interpretation of Proton Generalized Polarizabilities
We extend the recently developed formalism to extract light-front quark
charge densities from nucleon form factor data to the deformations of these
quark charge densities when applying an external electric field. We show that
the resulting induced polarizations can be extracted from proton generalized
polarizabilities. The available data for the generalized electric
polarizabilitiy of the proton yield a pronounced structure in its induced
polarization at large transverse distances, which will be pinned down by
forthcoming high precision virtual Compton scattering experiments.Comment: 4 pages, 4 figures, reference added, minor typos correcte
Spin/Orbital Structure of the Nucleon from Wigner Distributions
We present results for the Wigner distributions of the nucleon, which provide multidimensional images of the quark distributions in the phase space and can be seen as the mother distributions of the standard generalized and transverse-momentum dependent parton distributions. We discuss the general features of these distributions within a light-front constituent quark model, emphasizing the new information that can be deduced about the spin-orbit correlations of the quarks in the nucleon
Twist-2 Generalized TMDs and the Spin/Orbital Structure of the Nucleon
Generalized transverse-momentum dependent parton distributions (GTMDs) encode
the most general parton structure of hadrons. Here we focus on two twist-2
GTMDs which are denoted by and in parts of the literature.
As already shown previously, both GTMDs have a close relation to orbital
angular momentum of partons inside a hadron. However, recently even the mere
existence of and has been doubted. We explain why this
claim does not hold. We support our model-independent considerations by
calculating the two GTMDs in the scalar diquark model and in the quark-target
model, where we also explicitly check the relation to orbital angular momentum.
In addition, we compute and at large transverse momentum in
perturbative Quantum Chromodynamics and show that they are nonzero.Comment: 29 pages, 6 figures; two clarifications and a reference added;
version to appear in Phys. Rev.
Delta electromagnetic form factors and quark transverse charge densities from lattice QCD
We discuss the techniques to extract the electromagnetic Delta form factors
in Lattice QCD. We evaluate these form factors using dynamical fermions with
smallest pion mass of about 350 MeV. We pay particular attention to the
extraction of the electric quadrupole form factor that signals a deformation of
the Delta. The magnetic moment of the is extrapolated using a chiral
effective field theory. Using the form factors we evaluate the transverse
density distributions in the infinite momentum frame showing deformation in the
Delta.Comment: 9 pages, 6 figures, Invited Talk at the 6th International Workshop on
Chiral Dynamics, CD09, July 6-10, 2009, Bern, Switzerlan
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