42 research outputs found
Transverse single spin asymmetry in elastic electron-proton scattering
We discuss the two-photon exchange contribution to observables which involve lepton helicity flip in elastic lepton-nucleon scattering. This contribution is accessed through the single spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions
Renormalization of quark propagator, vertex functions and twist-2 operators from twisted-mass lattice QCD at =4
We present a precise non-perturbative determination of the renormalization
constants in the mass independent RI'-MOM scheme. The lattice implementation
uses the Iwasaki gauge action and four degenerate dynamical twisted mass
fermions. The gauge configurations are provided by the ETM Collaboration.
Renormalization constants for scalar, pseudo-scalar, vector and axial
operators, as well as the quark propagator renormalization, are computed at
three different values of the lattice spacing, two volumes and several twisted
mass parameters. The method we developed allows for a precise cross-check of
the running, thanks to the particular proper treatment of hypercubic artifacts.
Results for the twist-2 operator are also presented.Comment: 20 pages, 20 figures, submitted to Phys. Rev.
Renormalization constants for twisted mass QCD
We summarize recent non-perturbative results obtained for the renormalization
constants computed in the RI'-MOM scheme for twisted mass
QCD. Our implementation employs the Iwasaki gauge action and four dynamical
degenerate twisted mass fermions. Renormalization constants for scalar,
pseudo-scalar, vector and axial operators, as well as the quark propagator
renormalization, are computed at three different values of the lattice spacing,
two different volumes and several values of the twisted mass. Our method allows
for a precise cross-check of the running, because of the particular proper
treatment of the hypercubic artifacts. Preliminary results for twist-2
operators are also presented
The Recoil Proton Polarization: a new discriminative DVCS observable
Generalized parton distributions describe the correlations between the
longitudinal momentum and the transverse position of quarks and gluons in a
nucleon. They can be constrained by measuring photon leptoproduction
observables, arising from the interference between Bethe-Heitler and Deeply
virtual Compton scattering processes. At leading-twist/leading-order, the
amplitude of the latter is parameterized by complex integrals of the GPDs {H,
E, \~H, \~E} . As data collected on an unpolarized or longitudinally polarized
target constrains H and \~H, E is poorly known as it requires data collected
with a transversely polarized target, which is very challenging to implement in
fixed target experiments. The only alternative considered so far has been DVCS
on a neutron with a deuterium target, while assuming isospin symmetry and
absence of final-state interactions. Today, we introduce the polarization of
the recoil proton as a new DVCS observable, highly sensitive to E, which
appears feasible for an experimental study at a high-luminosity facility such
as Jefferson Lab
Excluded Volume Effects on Cold Neutron Star Phenomenology
Observable properties of neutron stars are studied within a hadronic equation
of state derived from the quark level. The effect of short-range repulsion is
incorporated within the excluded volume framework. It is found that one can
sustain neutron stars with masses as large as 2.2 even including
hyperons in equilibrium, while producing radii and tidal
deformabilities consistent with current constraints
Baryon masses with dynamical twisted mass fermions
We present results on the mass of the nucleon and the using two
dynamical degenerate twisted mass quarks. The evaluation is performed at four
quark masses corresponding to a pion mass in the range of 690-300 MeV on
lattices of size 2.1 fm and 2.7 fm. We check for cutoff effects by evaluating
these baryon masses on lattices of spatial size 2.1 fm with lattice spacings
fm and fm, determined from
the pion sector and find them to be within our statistical errors. Lattice
results are extrapolated to the physical limit using continuum chiral
perturbation theory.
The nucleon mass at the physical point provides a determination of the
lattice spacing. Using heavy baryon chiral perturbation theory at we find fm, with a systematic error due to
the chiral extrapolation estimated to be about the same as the statistical
error. This value of the lattice spacing is in good agreement with the value
determined from the pion sector. We check for isospin breaking in the
-system. We find that and are almost
degenerate pointing to small flavor violating effects.Comment: 7 pages, 9 figures. Talk presented at the XXV International Symposium
on Lattice Field Theory, July 30 - August 4 2007, Regensburg, German
The role of nucleon structure in finite nuclei
The quark-meson coupling model, based on a mean field description of
non-overlapping nucleon bags bound by the self-consistent exchange of ,
and mesons, is extended to investigate the properties of finite
nuclei. Using the Born-Oppenheimer approximation to describe the interacting
quark-meson system, we derive the effective equation of motion for the nucleon,
as well as the self-consistent equations for the meson mean fields. The model
is first applied to nuclear matter, after which we show some initial results
for finite nuclei.Comment: The revised version. This is tar, compressed and uuencoded (including
3 tables and 8 figures). 45 page
Superheavy Nuclei in the Quark-Meson-Coupling Model
We present a selection of the first results obtained in a comprehensive calculation of ground state properties of even-even superheavy nuclei in the region of 96 < Z < 136 and 118 < N < 320 from the Quark-Meson-Coupling model (QMC). Ground state binding energies, the neutron and proton number dependence of quadrupole deformations and Qα values are reported for even-even nuclei with 100 < Z < 136 and compared with available experimental data and predictions of macro-microscopic models. Predictions of properties of nuclei, including Qα values, relevant for planning future experiments are presented
Superheavy Nuclei in the Quark-Meson-Coupling Model
We present a selection of the first results obtained in a comprehensive calculation of ground state properties of even-even superheavy nuclei in the region of 96 < Z < 136 and 118 < N < 320 from the Quark-Meson-Coupling model (QMC). Ground state binding energies, the neutron and proton number dependence of quadrupole deformations and Qα values are reported for even-even nuclei with 100 < Z < 136 and compared with available experimental data and predictions of macro-microscopic models. Predictions of properties of nuclei, including Qα values, relevant for planning future experiments are presented
Chromodynamique quantique sur réseau et propriétés du nucléon
L objet de cette thèse est le calcul ab-initio des propriétés du nucléon en partant de la théorie microscopique de l interaction forte, la chromodynamique antique (QCD). Celle-ci est discrétisée sur un réseau quadridimensionnel et les observables quantiques sont calculées par la méthode de l intégrale de chemin, comme expliqué dans les chapitres II et III. Dans le chapitre IV nous discutons les problèmes posés par la discrétisation des fermions et nous expliquons le choix retenu par nos calculs c est-à-dire la discrétisation à la Wilson avec masse twistée.Le calcul numérique de l intégrale de chemin est fait par la méthode de Monte Carlo avec échantillonnage préférentiel. L algorythme Hybrid Monte Carlo , basé sur la dynamique moléculaire, est présenté dans le chapitre V ainsi que la méthode de résolution de grands systèmes linéaires creux qui apparaissent dans le calcul des observables. Ce chapitre présente aussi les aspects informatiques du problème, c est-à-dire le parallélisme massif ainsi que les caractéristiques des machines utilisées. Dans le chapitre VI nous expliquons la méthodologie suivie pour pour la production des ensembles représentatifs de configuration de jauge. La mise en œuvre et le contrôle de cette production est une part importante du travail effectué pendant cette thèse. Les deux derniers chapitres sont consacrés au calcul proprement dit des observables et à la présentation des résultats. Dans la conclusion nous faisons le point des calculs de QCD sur réseau et nous discutons de l évolution du domaine dans la perspective des nouveaux moyens de calculs et des développements théoriques récents.The goal of this thesis is to compute from first principles nucleon properties, starting from the microscopic theory of strong interaction, quantum chromodynamics (QCD). It uses a four-dimensional lattice on which theory is discretized and quantum observables are computed through path integral techniques, as explained in chapters II and III. In chapter IV we discuss problems faced when fermions are taken into account and we present the choice for our computations ie a discretization à la Wilson plus an additional twisted mass. The numerical evaluation of path integrals is done by Monte Carlo methods with importance sampling. The Hybrid Monte Carlo algorithm, based on molecular dynamics, is presented in chapter V together with a method to solve large sparse linear systems necessary to compute observables. This chapter also describes computer science details of the problem which are the use of massive parallel processing and some characteristics of computers used. In chapter VI we explain how the production of representative samples of gauge configuration is performed. This step and its control is an important part of the work done during this thesis. The last two chapters are devoted to the computation of observables and to the presentation of results. The main technical difficulty which is to solve for quark propagators has been performed by using available processor farms at their best. A good part of this work has been focused on this. To conclude we comment on the status of the calculations and we discuss the evolution of the field in the perspective of new computing facilities and recent theoretical progress.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF