152 research outputs found
Flexible Parametrization of Generalized Parton Distributions: The Chiral-Odd Sector
We present a physically motivated parameterization of the chiral-odd
generalized parton distributions. The parametrization is an extension of our
previous one in the chiral-even sector which was based on the reggeized diquark
model. While for chiral even generalized distributions a quantitative fit with
uncertainty estimation can be performed using deep inelastic scattering data,
nucleon electromagnetic, axial and pseudoscalar form factors measurements, and
all available deeply virtual Compton scattering data, the chiral-odd sector is
far less constrained. While awaiting the analysis of measurements on
pseudoscalar mesons exclusive electroproduction which are key for the
extraction of chiral odd GPDs, we worked out a connection between the
chiral-even and chiral-odd reduced helicity amplitudes using Parity
transformations. The connection works for a class of models including
two-components models. This relation allows us to estimate the size of the
various chiral odd contributions and it opens the way for future quantitative
fits.Comment: 35 pages, 18 figures, text changes, corrected typos, added fig.
Methane-Rare Gas Interaction Potentials from Scattering Experiments
By using total differential cross sections with resolved diffraction oscillations and absolute integral cross sections with glory oscillations, a new improved spherically averaged potential surface for Ne-CH4 is determined with a potential well depth of ε = 5.50 meV and a minimum distance of Rm = 3.78 Å. A slightly improved parameter set is also given for the isotropic Ar-CH4 interaction. By assuming the same realistic reduced potential form as was found for Ne-CH4 and Ar-CH4, the isotropic parts of the potential for Kr-CH4 and Xe-CH4 are obtained from absolute integral cross sections
Generalized parton distributions, the hunt for quark orbital momenta
The Generalized Parton Distributions (GPDs) are the appropriate framework for
a universal description of the partonic structure of the nucleon. They
characterize the dynamics of quarks and gluons inside the nucleon and
consequently contain information about the spin of the nucleon. The current
experimental knowledge about GPDs is reviewed with the emphasis on the
determination of E^q(Q^2,x,xi,t), the least known and constrained GPD, of
particular importance in the nucleon spin puzzle. The perspectives of this
experimental program are also addressed.Comment: 6 pages, 2 figures Proceedings of the XVIIIth Symposium on Spin
Physics, Charlottesville (Virginia, USA), October 6-11,200
Transverse spin in exclusive experiments
We analyze a number of open questions in the application of the concept of generalized parton distribution to various high-energy exclusive processes. In particular we discuss the feasibility of global fits of the exclusive data, and provide a recursive procedure based on a physically motivated parametrization. By fixing the parameters to the Deeply Virtual Compton Scattering data we make predictions
for chiral odd quantities, including transversity. All of our predictions include theoretical error
Generalized parton distributions: Status and perspectives
We summarize recent developments in understanding the concept of generalized
parton distributions (GPDs), its relation to nucleon structure, and its
application to high-Q2 electroproduction processes. Following a brief review of
QCD factorization and transverse nucleon structure, we discuss (a) new
theoretical methods for the analysis of deeply-virtual Compton scattering
(t-channel-based GPD parametrizations, dispersion relations); (b) the
phenomenology of hard exclusive meson production (experimental tests of
dominance of small-size configurations, model-independent comparative studies);
(c) the role of GPDs in small-x physics and pp scattering (QCD dipole model,
central exclusive diffraction). We emphasize the usefulness of the transverse
spatial (or impact parameter) representation for both understanding the
reaction mechanism in hard exclusive processes and visualizing the physical
content of the GPDs.Comment: 10 pages, 6 figures. Proceedings of SPIN2008, University of Virginia,
October 6-11, 200
The strong coupling constant at large distances
In this paper we discuss effective strong coupling constants. Those are well
behaved in the low-Q^2 domain, contrarily to alpha_s from pQCD. We present an
extraction of an effective strong coupling constant from Jefferson Lab
polarized data at intermediate and low Q^2. We also show how these data,
together with spin sum rules, allow us to obtain the effective coupling
constant over the entire Q^2 range. We then discuss the relation between the
experimentally extracted coupling constant and theoretical calculations at low
Q^2. We conclude on the importance of such study for the application of the
AdS/CFT correspondence to QCD.Comment: Proceedings for Spin2008 symposium. v2: added 2 referecences re.
infrared fixed poin
Future Investigations of the Flavor Dependence of Sea Quark Helicities at STAR
The flavor dependence of polarized and unpolarized quark distributions in the
nucleon can lead to insights into the formation of the sea. Drell-Yan
measurements have pointed to flavor asymmetries in the unpolarized
distributions. Collisions at sqrt{s}=500 GeV with polarized protons at RHIC
will soon allow investigations of the flavor separated polarized quark
distributions via W production to complement measurements from semi-inclusive
DIS. We report on STAR's current plans, tracking upgrade, and expected
sensitivities.Comment: 4 pages, to appear in the proceedings of the 18th International Spin
Physics Symposium (Spin08), Charlottesville, Virginia, USA, October 6 - 11,
200
Parton Distributions in the Impact Parameter Space
Parton distributions in impact parameter space, which are obtained by Fourier
transforming GPDs, exhibit a significant deviation from axial symmetry when the
target and/or quark is transversely polarized. In combination with the final
state interactions, this transverse deformation provides a natural mechanism
for naive-T odd transverse single-spin asymmetries in semi-inclusive DIS. The
deformation can also be related to the transverse force acting on the active
quark in polarized DIS at higher twist.Comment: 9 pages, Proceedings Spin 200
Deep inelastic scattering from A=3 nuclei and the neutron structure function
We present a comprehensive analysis of deep inelastic scattering from He-3
and H-3, focusing in particular on the extraction of the free neutron structure
function, F_2^n. Nuclear corrections are shown to cancel to within 1-2% for the
isospin-weighted ratio of He-3 to H-3 structure functions, which leads to more
than an order of magnitude improvement in the current uncertainty on the
neutron to proton ratio F_2^n/F_2^p at large x. Theoretical uncertainties
originating from the nuclear wave function, including possible non-nucleonic
components, are evaluated. Measurement of the He-3 and H-3 structure functions
will, in addition, determine the magnitude of the EMC effect in all A < 4
nuclei.Comment: 40 pages, 12 figures, to appear in Phys. Rev.
GPDs, their relationships with TMDs and related topics
Generalized Parton Distributions in the chiral odd sector are evaluated by examining their contribution to the quark proton helicity amplitudes, and by using Parity transformations to establish relations with the chiral even sector. The relations we find are valid in a spectator model where the recoiling system has total spin S=0,1. These relations allow us to fix the parameters for the chiral odd generalized parton distributions using results from a global analysis of unpolarized and longitudinally polarized experimental data in the kinematical region of intermediate Bjorken x and for Q2 in the multi-GeV region. Quantitative relations between chiral odd generalized parton distributions and transverse momentum dependent
distributions are given, and their extension beyond spectator models is discussed. Contributions to the Generalized Transverse Momentum Distributions are obtained
and considered in relation to observables. The related question about recoiling baryon polarizations is discussed
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