785 research outputs found
Radiative production of the Lambda(1405) resonance in K collisions on protons and nuclei
We have carried a theoretical study of the K^- p\to M B \gamma reaction with
M B = K^-p, \bar{K}^0 n, \pi^- \Sigma^+, \pi^+ \Sigma^-, \pi^0 \Sigma^0, \pi^0
\Lambda, for K^- lab. momenta between 200 and 500 MeV/c, using a chiral unitary
approach for the strong K^-p interaction with its coupled channels. The
\Lambda(1405) resonance, which is generated dynamically in this approach, shows
up clearly in the d\sigma/dM_I spectrum, providing new tests for chiral
symmetry and the unitary approach, as well as information regarding the nature
of the resonance. The photon detection alone, summing all channels, is shown to
reproduce quite accurately the strength and shape of the \Lambda(1405)
resonance. Analogous reactions in nuclei can provide much information on the
properties of this resonance in a nuclear medium.Comment: 11 pages, 3 postscripts figure
Dynamical meson-baryon resonances with chiral Lagrangians
The s-wave meson-baryon interaction is studied using the lowest-order chiral
Lagrangian in a unitary coupled-channels Bethe-Salpeter equation. In the
strangeness sector the low-energy dynamics leads to the
dynamical generation of the as a state, along with
a good description of the scattering observables. At higher energies,
the is also found to be generated dynamically as a
quasibound state for the first time. For strangeness S=0, it is the
resonance that emerges from the coupled-channels equations,
leading to a satisfactory description of meson-baryon scattering observables in
the energy region around the . We speculate on the possible
dynamical generation of resonances within the chiral sector as
or quasibound states.Comment: 8 pages, 5 figures, Talk given at NSTAR2001, Workshop on the Physics
of Excited Nucleons, Mainz (Germany), March 7-10, to be published in World
Scientifi
Photoproduction of meson and baryon resonances in a chiral unitary approach
By means of a coupled channel non-perturbative unitary approach, it is
possible to extend the strong constrains of Chiral Perturbation Theory to
higher energies. In particular, it is possible to reproduce the lowest lying
resonances in meson-meson scattering up to 1.2 GeV using the parameters of the
O(p^2) and O(p^4) Chiral Lagrangian. The meson baryon sector can also be
tackled along similar lines. We report on an update of these results showing
some examples of photon induced reactions where the techniques have been
recently applied.Comment: Contribution to the Erice Summer School of Nuclear Physics, 21th
course: Electromagnetic Probes and the Structure of Hadrons and Nuclei
September 17th - 25th, 1999, Erice/Sicily/Ital
High nuclear polarization of helium-3 at low and high pressure by metastability exchange optical pumping at 1.5 Tesla
We perform metastability exchange optical pumping of helium-3 in a strong
magnetic field of 1.5 T. The achieved nuclear polarization, from 80% at 1.33
mbar to 25% at 67 mbar, shows a substantial improvement at high pressures with
respect to standard low-field optical pumping. The specific mechanisms of
metastability exchange optical pumping at high field are investigated,
advantages and intrinsic limitations are discussed. From a practical point of
view, our results open the way to alternative technological solutions for
polarized helium-3 applications and in particular for magnetic resonance
imaging of human lungs.Comment: accepted for publication in Europhysics Letter
Flexible construction of hierarchical scale-free networks with general exponent
Extensive studies have been done to understand the principles behind
architectures of real networks. Recently, evidences for hierarchical
organization in many real networks have also been reported. Here, we present a
new hierarchical model which reproduces the main experimental properties
observed in real networks: scale-free of degree distribution (frequency
of the nodes that are connected to other nodes decays as a power-law
) and power-law scaling of the clustering coefficient
. The major novelties of our model can be summarized as
follows: {\it (a)} The model generates networks with scale-free distribution
for the degree of nodes with general exponent , and arbitrarily
close to any specified value, being able to reproduce most of the observed
hierarchical scale-free topologies. In contrast, previous models can not obtain
values of . {\it (b)} Our model has structural flexibility
because {\it (i)} it can incorporate various types of basic building blocks
(e.g., triangles, tetrahedrons and, in general, fully connected clusters of
nodes) and {\it (ii)} it allows a large variety of configurations (i.e., the
model can use more than copies of basic blocks of nodes). The
structural features of our proposed model might lead to a better understanding
of architectures of biological and non-biological networks.Comment: RevTeX, 5 pages, 4 figure
NN coupling and two-pion photoproduction on the nucleon
Effects of non-resonant photoproductions arising from two different
couplings are investigated in the reaction. We find that
the pseudoscalar (PS) coupling is generally preferable to the
pseudovector (PV) coupling and particularly the total cross sections
are successfully described by the model with the PS coupling. In order
to see the difference between the two couplings, we also show the results of
invariant mass spectra and helicity-dependent cross sections in various isospin
channels calculated with the PS and PV couplings.Comment: 35 pages, 11 figures, minor changes and version to be published in
Phys.Rev.
Magnetic moments of the Lambda(1405) and Lambda(1670) resonances
By using techniques of unitarized chiral perturbation theory, where the
and resonances are dynamically generated, we
evaluate the magnetic moments of these resonances and their transition magnetic
moment. The results obtained here differ appreciably from those obtained with
existing quark models. The width for the transition is also evaluated leading to a branching ratio of the order
of .Comment: 19 pags., 8 fig
Analysis of CLAS data on double charge pion electroproduction
Recent developments in phenomenological analysis of the CLAS data on 2
electroproduction are presented. The contributions from isobar channels and
, electrocouplings at from 0.25 to 0.6
GeV were determined from the analysis of comprehensive data on differential
and fully integrated 2 cross sections. Experiment Numbers: E94-005 Group:
Hall BComment: Paper compiled for NSTAR2007 meetin
Dynamical Generation of Hyperon Resonances
In this talk we report on how, using a chiral unitary approach for the
meson--baryon interactions, two octets of baryon states and a
singlet are generated dynamically, resulting in the case of strangeness
in two poles of the scattering matrix close to the nominal
resonance. We suggest experiments which could show evidence for the existence
of these states.Comment: Invited talk in the VIII International Conference on Hypernuclei and
Strange Particle Physic
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