2,224 research outputs found
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
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
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
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models
[abridged] Previous analyses of lithium abundances in main sequence and red
giant stars have revealed the action of mixing mechanisms other than convection
in stellar interiors. Beryllium abundances in stars with lithium abundance
determinations can offer valuable complementary information on the nature of
these mechanisms. Our aim is to derive beryllium abundances along the whole
evolutionary sequence of an open cluster, IC 4651. These Be abundances are used
together with previously determined Li abundances, in the same sample stars, to
investigate the mixing mechanisms in a range of stellar masses and evolutionary
stages. New beryllium abundances are determined from high-resolution, high
signal-to-noise UVES spectra using spectrum synthesis and model atmospheres.
The careful synthetic modelling of the Be lines region is used to calculate
reliable abundances in rapidly rotating stars. The observed behavior of Be and
Li is compared to theoretical predictions from stellar models including
rotation-induced mixing, internal gravity waves, atomic diffusion, and
thermohaline mixing. Beryllium is detected in all the main sequence and
turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip
stars, but was not detected in the red giants. Confirming previous results, we
find that the Li dip is also a Be dip, although the depletion of Be is more
modest than that of Li in the corresponding effective temperature range. For
post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung
gap than expected from classical predictions as does the Li dilution. A clear
dispersion in the Be abundances is also observed. Theoretical stellar models
including the hydrodynamical transport processes mentioned above are able to
reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio
Testing the LCDM model (and more) with the time evolution of the redshift
With the many ambitious proposals afoot for new generations of very large
telescopes, along with spectrographs of unprecedented resolution, there arises
the real possibility that the time evolution of the cosmological redshift may,
in the not too distant future, prove to be a useful tool rather than merely a
theoretical curiosity. Here I contrast this approach with the standard
cosmological procedure based on the luminosity (or any other well-defined)
distance. I then show that such observations would not only provide a direct
measure of all the associated cosmological parameters of the LCDM model, but
would also provide wide-ranging internal consistency checks. Further, in a more
general context, I show that without introducing further time derivatives of
the redshift one could in fact map out the dark energy equation of state should
the LCDM model fail. A consideration of brane-world scenarios and interacting
dark energy models serves to emphasize the fact that the usefulness of such
observations would not be restricted to high redshifts.Comment: In final form as to appear in Physical Review D. 12 pages 6 figure
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
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
Naive time-reversal odd phenomena in semi-inclusive deep-inelastic scattering from light-cone constituent quark models
We present results for leading-twist azimuthal asymmetries in semi-inclusive
lepton-nucleon deep-inelastic scattering due to naively time-reversal odd
transverse-momentum dependent parton distribution functions from the light-cone
constituent quark model. We carefully discuss the range of applicability of the
model, especially with regard to positivity constraints and evolution effects.
We find good agreement with available experimental data from COMPASS and
HERMES, and present predictions to be tested in forthcoming experiments at
Jefferson Lab.Comment: 10 pages, 7 figures, discussion of evolution effects extended, to
appear in Phys.Rev.
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
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