21,010 research outputs found
Quark Cluster Model Study of Isospin-Two Dibaryons
Based on a quark cluster model for the non-strange sector that reproduces
reasonably well the nucleon-nucleon system and the excitation of the
isobar, we generate a nucleon- interaction and present the predictions
for the several isospin two channels. The only attractive channels are
and , but not attractive enough to generate a resonance. If a resonance is
artificially generated and is required to have the observed experimental mass,
then our model predicts a width that agrees with the experimental result.Comment: 12 pages, 5 poscript figures available under request. To appear in
Phys. Rev.
Does the quark cluster model predict any isospin two dibaryon resonance?
We analyze the possible existence of a resonance in the channel
with isospin two by means of nucleon- interactions based on the
constituent quark model. We solve the bound state and the scattering problem
using two different potentials, a local and a non-local one. The non-local
potential results to be the more attractive, although not enough to generate
the experimentally predicted resonance.Comment: 9 pages in Latex (revtex), 2 eps figures available under reques
Albedos of Main-Belt Comets 133P/Elst-Pizarro and 176P/LINEAR
We present the determination of the geometric R-band albedos of two main-belt
comet nuclei based on data from the Spitzer Space Telescope and a number of
ground-based optical facilities. For 133P/Elst-Pizarro, we find an albedo of
p_R=0.05+/-0.02 and an effective radius of r_e=1.9+/-0.3 km (estimated
semi-axes of a~2.3 km and b~1.6 km). For 176P/LINEAR, we find an albedo of
p_R=0.06+/-0.02 and an effective radius of r_e=2.0+/-0.2 km (estimated
semi-axes of a~2.6 km and b~1.5 km). In terms of albedo, 133P and 176P are
similar to each other and are typical of other Themis family asteroids, C-class
asteroids, and other comet nuclei. We find no indication that 133P and 176P are
compositionally unique among other dynamically-similar (but inactive) members
of the Themis family, in agreement with previous assertions that the two
objects most likely formed in-situ. We also note that low albedo (p_R<0.075)
remains a consistent feature of all cometary (i.e., icy) bodies, whether they
originate in the inner solar system (the main-belt comets) or in the outer
solar system (all other comets).Comment: 11 pages, 3 figures, accepted for publication in ApJ
A family of complex potentials with real spectrum
We consider a two-parameter non hermitean quantum-mechanical hamiltonian that
is invariant under the combined effects of parity and time reversal
transformation. Numerical investigation shows that for some values of the
potential parameters the hamiltonian operator supports real eigenvalues and
localized eigenfunctions. In contrast with other PT symmetric models, which
require special integration paths in the complex plane, our model is integrable
along a line parallel to the real axis.Comment: Six figures and four table
Poisson approximations for the Ising model
A -dimensional Ising model on a lattice torus is considered. As the size
of the lattice tends to infinity, a Poisson approximation is given for the
distribution of the number of copies in the lattice of any given local
configuration, provided the magnetic field tends to and the
pair potential remains fixed. Using the Stein-Chen method, a bound is given
for the total variation error in the ferromagnetic case.Comment: 25 pages, 1 figur
Partially ordered models
We provide a formal definition and study the basic properties of partially
ordered chains (POC). These systems were proposed to model textures in image
processing and to represent independence relations between random variables in
statistics (in the later case they are known as Bayesian networks). Our chains
are a generalization of probabilistic cellular automata (PCA) and their theory
has features intermediate between that of discrete-time processes and the
theory of statistical mechanical lattice fields. Its proper definition is based
on the notion of partially ordered specification (POS), in close analogy to the
theory of Gibbs measure. This paper contains two types of results. First, we
present the basic elements of the general theory of POCs: basic geometrical
issues, definition in terms of conditional probability kernels, extremal
decomposition, extremality and triviality, reconstruction starting from
single-site kernels, relations between POM and Gibbs fields. Second, we prove
three uniqueness criteria that correspond to the criteria known as bounded
uniformity, Dobrushin and disagreement percolation in the theory of Gibbs
measures.Comment: 54 pages, 11 figures, 6 simulations. Submited to Journal of Stat.
Phy
Roper Excitation in Alpha-Proton Scattering
We study the Roper excitation in the reaction. We consider
all processes which may be relevant in the Roper excitation region, namely,
Roper excitation in the target, Roper excitation in the projectile, and double
excitation processes. The theoretical investigation shows that the
Roper excitation in the proton target mediated by an isoscalar exchange is the
dominant mechanism in the process. We determine an effective isoscalar
interaction by means of which the experimental cross section is well
reproduced. This should be useful to make predictions in related reactions and
is a first step to construct eventually a microscopic
transition potential, for which the present reaction does not offer enough
information.Comment: Latex 17 pages; figures available by request; Phys. Rev. C in prin
Characterization of Carbon-Contaminated B4C-Coated Optics after Chemically Selective Cleaning with Low-Pressure RF Plasma
Boron carbide (B4C) is one of the few materials that is expected to be mostly
resilient with respect to the extremely high brilliance of the photon beam
generated by free electron lasers (FELs) and is thus of considerable interest
for optical applications in this field. However, as in the case of many other
optics operated at modern light source facilities, B4C-coated optics are
subject to ubiquitous carbon contaminations. These contaminations represent a
serious issue for the operation of high performance FEL beamlines due to severe
reduction of photon flux, beam coherence, creation of destructive interference,
and scattering losses. A variety of B4C cleaning technologies were developed at
different laboratories with varying success. We present a study regarding the
low-pressure RF plasma cleaning of carbon contaminated B4C test samples via
inductively coupled O2/Ar, H2/Ar, and pure O2 RF plasma produced following
previous studies using the same IBSS GV10x downstream plasma source. Results
regarding the chemistry, morphology as well as other aspects of the B4C optical
coating before and after the plasma cleaning are reported. We conclude from
these comparative plasma processes that pure O2 feedstock plasma only exhibits
the required chemical selectivity for maintaining the integrity of the B4C
optical coating.Comment: 27 pages, 15 figure
Systematic perturbation calculation of integrals with applications to physics
In this paper we generalize and improve a method for calculating the period
of a classical oscillator and other integrals of physical interest, which was
recently developed by some of the authors. We derive analytical expressions
that prove to be more accurate than those commonly found in the literature, and
test the convergence of the series produced by the approach.Comment: 11 pages, 5 figure
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