4,507 research outputs found
Single Spin Asymmetries in Semi-Inclusive Electroproduction: Access to Transversity
We discuss the quark transversity distribution function and a possible way to
access it through the measurement of single spin azimuthal asymmetry in
semi-inclusive single pion electroproduction on a transversely polarized
target.Comment: 5 pages, Latex using aipproc.sty (included), to appear in proceedings
of "Second Workshop on Physics with an Electron Polarized Light Ion
Collider", Sept. 14-16, 2000, MIT, Cambridge, US
Electromagnetic Form Factors in the hypercentral CQM
We report on the recent results of the hypercentral Constituent Quark Model
(hCQM). The model contains a spin independent three-quark interaction which is
inspired by Lattice QCD calculations and reproduces the average energy values
of the SU(6) multiplets. The splittings are obtained with a SU(6)-breaking
interaction, which can include also an isospin dependent term. Concerning
Constituent Quark models, we have shown for the first time that the decreasing
of the ratio of the elastic form factors of the proton is due to relativistic
effects using relativistic corrections to the e.m. current and boosts. Now the
elastic nucleon form factors have been recalculated, using a relativistic
version of the hCQM and a relativistic quark current showing a very detailed
reproduction of all the four form factor existing data over the complete range
of 0-4 . Futhermore, the model has been used for predictions concerning
the electromagnetic transverse and longitudinal transition form factors giving
a good description of the medium behaviour. We show that the
discrepancies in the reproduction of the helicity amplitudes at low are
due to pion loops. We have calculated the helicity amplitudes for all the 3 and
4 star resonances opening the possibility of application to the evaluation of
cross sections.Comment: 5 pages, 7 figures, Invited talk at the ICTP 4th International
Conference on Perspectives in Hadronic Physics, Trieste, Italy, 12-16 May
2003. Accepted by Eur. Phys. J.
Electroproduction of nucleon resonances
The unitary isobar model MAID has been extended and used for a partial wave
analysis of pion photo- and electroproduction in the resonance region W < 2
GeV. Older data from the world data base and more recent experimental results
from Mainz, Bates, Bonn and JLab for Q^2 up to 4.0 (GeV/c)^2 have been analyzed
and the Q^2 dependence of the helicity amplitudes have been extracted for a
series of four star resonances. We compare single-Q^2 analyses with a
superglobal fit in a new parametrization of Maid2003 together with predictions
of the hypercentral constituent quark model. As a result we find that the
helicity amplitudes and transition form factors of constituent quark models
should be compared with the analysis of bare resonances, where the pion cloud
contributions have been subtracted.Comment: 6 pages Latex including 5 figures, Invited talk at ICTP 4th
International Conference on Perspectives in Hadronic Physics, Trieste, Italy,
12-16 May 200
Sizing gold nanoparticles by optical extinction spectroscopy
The measurement of optical extinction is used to determine the size of nearly spherical gold nanoparticles suspended in solution, produced by a 'reverse micelles' process. The contrast between the maximum and the minimum in the extinction spectra around 450 and 520 nm shows a linear dependence with the mean radius of the gold particles less than 3 nm; however, the method can be used to size particles up to 7 nm. Experimental results for extinction spectra can be fitted by Mie's theory if the optical constants from bulk material values are modified by introducing the limitation of the mean free path due to collisions of conduction electrons with the boundary of the nanoparticles.Facultad de Ciencias ExactasCentro de Investigaciones Óptica
Sizing gold nanoparticles by optical extinction spectroscopy
The measurement of optical extinction is used to determine the size of nearly spherical gold nanoparticles suspended in solution, produced by a 'reverse micelles' process. The contrast between the maximum and the minimum in the extinction spectra around 450 and 520 nm shows a linear dependence with the mean radius of the gold particles less than 3 nm; however, the method can be used to size particles up to 7 nm. Experimental results for extinction spectra can be fitted by Mie's theory if the optical constants from bulk material values are modified by introducing the limitation of the mean free path due to collisions of conduction electrons with the boundary of the nanoparticles.Facultad de Ciencias ExactasCentro de Investigaciones Óptica
Investigation of single spin asymmetries in pi^{+} electroproduction
The azimuthal single target-spin asymmetries for pi^{+} production in
semi-inclusive deep inelastic scattering of leptons off longitudinally
polarized protons are evaluated using two main approaches available in the
literature. It is shown that the approximation where the twist-2 transverse
quark spin distribution in the longitudinally polarized nucleon is small enough
to be neglected leads to a consistent description of all existing asymmetries
observed by the HERMES experiment.Comment: 11 pages with 4 figures, LaTe
Effects of noise and confidence thresholds in nominal and metric Axelrod dynamics of social influence
We study the effects of bounded confidence thresholds and of interaction and
external noise on Axelrod's model of social influence. Our study is based on a
combination of numerical simulations and an integration of the mean-field
Master equation describing the system in the thermodynamic limit. We find that
interaction thresholds affect the system only quantitatively, but that they do
not alter the basic phase structure. The known crossover between an ordered and
a disordered state in finite systems subject to external noise persists in
models with general confidence threshold. Interaction noise here facilitates
the dynamics and reduces relaxation times. We also study Axelrod systems with
metric features, and point out similarities and differences compared to models
with nominal features. Metric features are used to demonstrate that a small
group of extremists can have a significant impact on the opinion dynamics of a
population of Axelrod agents.Comment: 15 pages, 12 figure
Perturbative contribution to the sin(phi) asymmetry in inclusive pi^{+} electroproduction
We consider the sin(phi) single target-spin asymmetry in deep-inelastic
pi^{+} inclusive electroproduction off a longitudinally polarized target. We
show that at larger transverse momentum of the outgoing hadron the evaluated
asymmetry decreases if one takes into account the first order alpha_S
perturbative contribution to the cross section, integrated over the azimuthal
angle. This leads to good agreement with recent HERMES data.Comment: 4 pages with one figure, LaTeX, typos corrected. Contribution to the
Proceedings of the QCD'00 Euroconference, Montpellier, 6-13th July 2000, to
appear in the Nucl. Phys. B (Proc. Suppl.
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