310 research outputs found
Symmetry Breaking in Linearly Coupled Dynamical Lattices
We examine one- and two-dimensional (1D and 2D) models of linearly coupled
lattices of the discrete-nonlinear-Schr{\"{o}}dinger type. Analyzing ground
states of the systems with equal powers in the two components, we find a
symmetry-breaking phenomenon beyond a critical value of the squared -norm.
Asymmetric states, with unequal powers in their components, emerge through a
subcritical pitchfork bifurcation, which, for very weakly coupled lattices,
changes into a supercritical one. We identify the stability of various solution
branches. Dynamical manifestations of the symmetry breaking are studied by
simulating the evolution of the unstable branches. The results present the
first example of spontaneous symmetry breaking in 2D lattice solitons. This
feature has no counterpart in the continuum limit, because of the collapse
instability in the latter case.Comment: 9 pages, 9 figures, submitted to Phys. Rev. E, Apr, 200
Beam Performance of Tracking Detectors with Industrially Produced GEM Foils
Three Gas-Electron-Multiplier tracking detectors with an active area of 10 cm
x 10 cm and a two-dimensional, laser-etched orthogonal strip readout have been
tested extensively in particle beams at the Meson Test Beam Facility at
Fermilab. These detectors used GEM foils produced by Tech-Etch, Inc. They
showed an efficiency in excess of 95% and spatial resolution better than 70 um.
The influence of the angle of incidence of particles on efficiency and spatial
resolution was studied in detail.Comment: 8 pages, 9 figures, accepted by Nuclear Instruments and Methods in
Physics Research
Dynamic coordinated control laws in multiple agent models
We present an active control scheme of a kinetic model of swarming. It has
been shown previously that the global control scheme for the model, presented
in \cite{JK04}, gives rise to spontaneous collective organization of agents
into a unified coherent swarm, via a long-range attractive and short-range
repulsive potential. We extend these results by presenting control laws whereby
a single swarm is broken into independently functioning subswarm clusters. The
transition between one coordinated swarm and multiple clustered subswarms is
managed simply with a homotopy parameter. Additionally, we present as an
alternate formulation, a local control law for the same model, which implements
dynamic barrier avoidance behavior, and in which swarm coherence emerges
spontaneously.Comment: 20 pages, 6 figure
Nucleon-Nucleon Optical Model for Energies to 3 GeV
Several nucleon-nucleon potentials, Paris, Nijmegen, Argonne, and those
derived by quantum inversion, which describe the NN interaction for T-lab below
300$ MeV are extended in their range of application as NN optical models.
Extensions are made in r-space using complex separable potentials definable
with a wide range of form factor options including those of boundary condition
models. We use the latest phase shift analyses SP00 (FA00, WI00) of Arndt et
al. from 300 MeV to 3 GeV to determine these extensions. The imaginary parts of
the optical model interactions account for loss of flux into direct or resonant
production processes. The optical potential approach is of particular value as
it permits one to visualize fusion, and subsequent fission, of nucleons when
T-lab above 2 GeV. We do so by calculating the scattering wave functions to
specify the energy and radial dependences of flux losses and of probability
distributions. Furthermore, half-off the energy shell t-matrices are presented
as they are readily deduced with this approach. Such t-matrices are required
for studies of few- and many-body nuclear reactions.Comment: Latex, 40 postscript pages including 17 figure
Measurement of the Absolute Differential Cross Section for np Elastic Scattering at 194 MeV
A tagged medium-energy neutron beam has been used in a precise measurement of
the absolute differential cross section for np back-scattering. The results
resolve significant discrepancies within the np database concerning the angular
dependence in this regime. The experiment has determined the absolute
normalization with 1.5% uncertainty, suitable to verify constraints of
supposedly comparable precision that arise from the rest of the database in
partial wave analyses. The analysis procedures, especially those associated
with evaluation of systematic errors in the experiment, are described in detail
so that systematic uncertainties may be included in a reasonable way in
subsequent partial wave analysis fits incorporating the present results.Comment: 22 pages, 21 figures, submitted for publication in Physical Review
Proton-proton scattering above 3 GeV/c
A large set of data on proton-proton differential cross sections, analyzing
powers and the double polarization parameter A_NN is analyzed employing the
Regge formalism. We find that the data available at proton beam momenta from 3
GeV/c to 50 GeV/c exhibit features that are very well in line with the general
characteristics of Regge phenomenology and can be described with a model that
includes the rho, omega, f_2, and a_2 trajectories and single Pomeron exchange.
Additional data, specifically for spin-dependent observables at forward angles,
would be very helpful for testing and refining our Regge model.Comment: 16 pages, 19 figures; revised version accepted for publication in
EPJ
Measurement of the Absolute np Scattering Differential Cross Section at 194 MeV
We describe a double-scattering experiment with a novel tagged neutron beam
to measure differential cross sections for np back-scattering to better than 2%
absolute precision. The measurement focuses on angles and energies where the
cross section magnitude and angle-dependence constrain the charged pion-nucleon
coupling constant, but existing data show serious discrepancies among
themselves and with energy-dependent partial wave analyses (PWA). The present
results are in good accord with the PWA, but deviate systematically from other
recent measurements.Comment: 4 pages, 4 figure
The nucleon-nucleon interaction
We review the major progress of the past decade concerning our understanding
of the nucleon-nucleon interaction. The focus is on the low-energy region
(below pion production threshold), but a brief outlook towards higher energies
is also given. The items discussed include charge-dependence, the precise value
of the coupling constant, phase shift analysis and high-precision NN
data and potentials. We also address the issue of a proper theory of nuclear
forces. Finally, we summarize the essential open questions that future research
should be devoted to.Comment: 42 pages, 12 figures, iopart.cls style; Topical Review prepared for
J. Phys. G: Nucl. Part. Phy
Clinical commissioning of intensity-modulated proton therapy systems: Report of AAPM Task Group 185
Proton therapy is an expanding radiotherapy modality in the United States and worldwide. With the number of proton therapy centers treating patients increasing, so does the need for consistent, high-quality clinical commissioning practices. Clinical commissioning encompasses the entire proton therapy system\u27s multiple components, including the treatment delivery system, the patient positioning system, and the image-guided radiotherapy components. Also included in the commissioning process are the x-ray computed tomography scanner calibration for proton stopping power, the radiotherapy treatment planning system, and corresponding portions of the treatment management system. This commissioning report focuses exclusively on intensity-modulated scanning systems, presenting details of how to perform the commissioning of the proton therapy and ancillary systems, including the required proton beam measurements, treatment planning system dose modeling, and the equipment needed
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