57 research outputs found
On the Origin of Model Relations among Transverse-Momentum Dependent Parton Distributions
Transverse-momentum dependent parton distributions (TMDs) are studied in the
framework of quark models. In particular, quark-model relations among TMDs are
reviewed, elucidating their physical origin in terms of the quark-spin
structure in the nucleon. The formal aspects of the derivation of these
relations are complemented with explicit examples, emphasizing how and to which
extent the conditions which lead to relations among TMDs are implemented in
different classes of quark models.Comment: 38 pages, 3 figures, 3 table
The angular momentum controversy: What's it all about and does it matter?
The general question, crucial to an understanding of the internal structure
of the nucleon, of how to split the total angular momentum of a photon or gluon
into spin and orbital contributions is one of the most important and
interesting challenges faced by gauge theories like Quantum Electrodynamics and
Quantum Chromodynamics. This is particularly challenging since all QED
textbooks state that such an splitting cannot be done for a photon (and a
fortiori for a gluon) in a gauge-invariant way, yet experimentalists around the
world are engaged in measuring what they believe is the gluon spin! This
question has been a subject of intense debate and controversy, ever since, in
2008, it was claimed that such a gauge-invariant split was, in fact, possible.
We explain in what sense this claim is true and how it turns out that one of
the main problems is that such a decomposition is not unique and therefore
raises the question of what is the most natural or physical choice. The
essential requirement of measurability does not solve the ambiguities and leads
us to the conclusion that the choice of a particular decomposition is
essentially a matter of taste and convenience. In this review, we provide a
pedagogical introduction to the question of angular momentum decomposition in a
gauge theory, present the main relevant decompositions and discuss in detail
several aspects of the controversies regarding the question of gauge
invariance, frame dependence, uniqueness and measurability. We stress the
physical implications of the recent developments and collect into a separate
section all the sum rules and relations which we think experimentally relevant.
We hope that such a review will make the matter amenable to a broader community
and will help to clarify the present situation.Comment: 96 pages, 11 figures, 5 tables, review prepared for Physics Report
Modeling the transverse momentum dependent parton distributions
We review quark model calculations of the transverse momentum dependent
parton distributions (TMDs). For the T-even TMDs, we discuss the physical
origin of model relations which hold in a large class of quark models. For the
T-odd TMDs we review results in a light-cone constituent quark model (CQM) with
the final state interaction effects generated via single-gluon exchange
mechanism. As phenomenological application, we show the good agreement between
results in the light-cone CQM and available experimental data for the Collins
asymmetry.Comment: 8 pages, 2 figures, 2 tables; invited talk at "The 4th Workshop on
Exclusive Reactions at High Momentum Transfer", Jefferson Lab., May 18-21,
201
Models for TMDs and numerical methods
We study the connection between the quark orbital angular momentum and the
pretzelosity transverse-momentum dependent parton distribution function. We
discuss the origin of this relation in quark models, identifying as key
ingredient for its validity the assumption of spherical symmetry for the
nucleon in its rest frame. Finally we show that the individual quark
contributions to the orbital angular momentum obtained from this relation can
not be interpreted as the intrinsic contributions, but include the contribution
from the transverse centre of momentum which cancels out only in the total
orbital angular momentum.Comment: 43 pages, 8 figures; proceedings of International School of Physics
"Enrico Fermi", Course CLXXX - "Three-dimensional Partonic Structure of the
Nucleon", 28 June - 8 July 2011, Varenna (Italy
Pretzelosity TMD and Quark Orbital Angular Momentum
We study the connection between the quark orbital angular momentum and the
pretzelosity transverse-momentum dependent parton distribution function. We
discuss the origin of this relation in quark models, identifying as key
ingredient for its validity the assumption of spherical symmetry for the
nucleon in its rest frame. Finally we show that the individual quark
contributions to the orbital angular momentum obtained from this relation can
not be interpreted as the intrinsic contributions, but include the contribution
from the transverse centre of momentum which cancels out only in the total
orbital angular momentum.Comment: 8 pages; references added, version to appear in Phys. Lett.
The partonic structure of the nucleon from generalized transverse momentum-dependent parton distributions
We discuss the general formalism for the calculation in light-front quark
models of the fully unintegrated, off-diagonal quark-quark correlator of the
nucleon, parametrized in terms of generalized transverse momentum dependent
parton distributions (GTMDs). By taking specific limits or projections, these
GTMDs yield various transverse-momentum dependent and generalized parton
distributions, thus providing a unified framework to simultaneously model
different observables. The corresponding distributions in impact-parameter
space are the Wigner functions which provide multidimensional images of the
quark distributions in phase space. We present results within a light-front
constituent quark model, discussing some of the complementary aspects encoded
in the different distributions and the relation to the quark orbital angular
momentum of the proton.Comment: 12 pages, 5 figures; invited talk at the international Conference
"Advanced Studies Institute Symmetries And Spin" (SPIN-Praha-2012) Prague,
July 1 - 8, 201
The quark orbital angular momentum from Wigner distributions and light-cone wave functions
We investigate the quark orbital angular momentum of the nucleon in the
absence of gauge-field degrees of freedom, by using the concept of the Wigner
distribution and the light-cone wave functions of the Fock state expansion of
the nucleon. The quark orbital angular momentum is obtained from the
phase-space average of the orbital angular momentum operator weighted with the
Wigner distribution of unpolarized quarks in a longitudinally polarized
nucleon. We also derive the light-cone wave function representation of the
orbital angular momentum. In particular, we perform an expansion in the nucleon
Fock state space and decompose the orbital angular momentum into the -parton
state contributions. Explicit expressions are presented in terms of the
light-cone wave functions of the three-quark Fock state. Numerical results for
the up and down quark orbital angular momenta of the proton are shown in the
light-cone constituent quark model and the light-cone chiral quark-soliton
model.Comment: 26 pages, 4 figure
Colloquium: Gravitational Form Factors of the Proton
The physics of the gravitational form factors of the proton, and their
understanding within quantum chromodynamics, has advanced significantly in the
past two decades through both theory and experiment. This Colloquium provides
an overview of this progress, highlights the physical insights unveiled by
studies of gravitational form factors, and reviews their interpretation in
terms of the mechanical properties of the proton.Comment: 22 pages, 15 figure
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