83 research outputs found
New explicit expressions for Dirac bilinears
We derive new explicit expressions for the Dirac bilinears based on a generic
representation of the massive Dirac spinors with canonical polarization. These
bilinears depend on a direction in Minkowski space which specifies the form
of dynamics. We argue that such a dependence is unavoidable in a relativistic
theory with spin, since it originates from Wigner rotation effects. Contrary to
most of the expressions found in the literature, our ones are valid for all
momenta and canonical polarizations of the spinors. As a by-product, we also
obtain a generic explicit expression for the covariant spin vector.Comment: 9 pages, version accepted in PR
Gauge symmetry and background independence: Should the proton spin decomposition be path independent?
Exploring the similarities between the Chen \emph{et al.} approach, where
physical and gauge degrees of freedom of the gauge potential are explicitly
separated, and the background field method, we provide an alternative point of
view to the proton spin decomposition issue. We show in particular that the
gauge symmetry can be realized in two different ways, and discuss the relations
between the concepts of path dependence, Stueckelberg dependence and background
dependence. Finally, we argue that path/Stueckelberg/background-dependent
decompositions of the proton spin are in principle measurable and therefore
physically meaningful.Comment: 10 pages, extended versio
The light-front gauge-invariant energy-momentum tensor
We provide for the first time a complete parametrization for the matrix
elements of the generic asymmetric, non-local and gauge-invariant canonical
energy-momentum tensor, generalizing therefore former works on the symmetric,
local and gauge-invariant kinetic energy-momentum tensor also known as the
Belinfante-Rosenfeld energy-momentum tensor. We discuss in detail the various
constraints imposed by non-locality, linear and angular momentum conservation.
We also derive the relations with two-parton generalized and
transverse-momentum dependent distributions, clarifying what can be learned
from the latter. In particular, we show explicitly that two-parton
transverse-momentum dependent distributions cannot provide any
model-independent information about the parton orbital angular momentum. On the
way, we recover the Burkardt sum rule and obtain similar new sum rules for
higher-twist distributions.Comment: 15 pages, 3 table
The nucleon spin decomposition: news and experimental implications
Recently, many nucleon spin decompositions have been proposed in the
literature, creating a lot of confusion. This revived in particular old
controversies regarding the measurability of theoretically defined quantities.
We propose a brief overview of the different decompositions, discuss the
sufficient requirements for measurability and stress the experimental
implications.Comment: 4 pages, contribution to the proceedings of the 13th International
Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU
2013), Sep 30-Oct 4, Rome, Ital
On the hadron mass decomposition
We argue that the standard decompositions of the hadron mass overlook
pressure effects, and hence should be interpreted with great care. Based on the
semiclassical picture, we propose a new decomposition that properly accounts
for these pressure effects. Because of Lorentz covariance, we stress that the
hadron mass decomposition automatically comes along with a stability
constraint, which we discuss for the first time. We show also that if a hadron
is seen as made of quarks and gluons, one cannot decompose its mass into more
than two contributions without running into trouble with the consistency of the
physical interpretation. In particular, the so-called quark mass and trace
anomaly contributions appear to be purely conventional. Based on the current
phenomenological values, we find that in average quarks exert a repulsive force
inside nucleons, balanced exactly by the gluon attractive force.Comment: 21 pages, 4 figure
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
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