6,808 research outputs found
Boxfishes (Teleostei: Ostraciidae) as a model system for fishes swimming with many fins: kinematics
Swimming movements in boxfishes were much more
complex and varied than classical descriptions indicated.
At low to moderate rectilinear swimming speeds
(<5 TL s^(-1), where TL is total body length), they were
entirely median- and paired-fin swimmers, apparently
using their caudal fins for steering. The pectoral and
median paired fins generate both the thrust needed for
forward motion and the continuously varied, interacting
forces required for the maintenance of rectilinearity. It
was only at higher swimming speeds (above 5 TL s^(-1)), when
burst-and-coast swimming was used, that they became
primarily body and caudal-fin swimmers. Despite their
unwieldy appearance and often asynchronous fin beats,
boxfish swam in a stable manner. Swimming boxfish used
three gaits. Fin-beat asymmetry and a relatively nonlinear
swimming trajectory characterized the first gait
(0–1 TL s^(-1)). The beginning of the second gait (1–3 TL s^(-1))
was characterized by varying fin-beat frequencies and
amplitudes as well as synchrony in pectoral fin motions.
The remainder of the second gait (3–5 TL s^(-1)) was
characterized by constant fin-beat amplitudes, varying finbeat
frequencies and increasing pectoral fin-beat
asynchrony. The third gait (>5 TL s^(-1)) was characterized
by the use of a caudal burst-and-coast variant. Adduction
was always faster than abduction in the pectoral fins.
There were no measurable refractory periods between
successive phases of the fin movement cycles. Dorsal and
anal fin movements were synchronized at speeds greater
than 2.5 TL s^(-1), but were often out of phase with pectoral
fin movements
Emergence of clusters: Halos, Efimov states, and experimental signals
We investigate emergence of halos and Efimov states in nuclei by use of a
newly designed model which combines self-consistent mean-field and three-body
descriptions. Recent interest in neutron heavy calcium isotopes makes Ca
(Ca+n+n) an ideal realistic candidate on the neutron dripline, and we
use it as a representative example that illustrates our broadly applicable
conclusions. By smooth variation of the interactions we simulate the crossover
from well-bound systems to structures beyond the threshold of binding, and find
that halo-configurations emerge from the mean-field structure for three-body
binding energy less than keV. Strong evidence is provided that Efimov
states cannot exist in nuclei. The structure that bears the most resemblance to
an Efimov state is a giant halo extending beyond the neutron-core scattering
length. We show that the observable large-distance decay properties of the wave
function can differ substantially from the bulk part at short distances, and
that this evolution can be traced with our combination of few- and many-body
formalisms. This connection is vital for interpretation of measurements such as
those where an initial state is populated in a reaction or by a beta-decay.Comment: 5 pages, 5 figures, under revie
Combined few-body and mean-field model for nuclei
The challenging nuclear many-body problem is discussed along with
classifications and qualitative descriptions of existing methods and models. We
present detailed derivations of a new method where cluster correlations
co-exist with an underlying mean-field described core-structure. The variation
of an antisymmetrized product of cluster and core wave functions and a given
nuclear interaction, provide sets of self-consistent equations of motion.
After the applications on dripline nuclei we discuss perspectives with
improvements and applications. In the conclusion we summarize while emphasizing
the merits of consistently treating both short- and large-distance properties,
few- and many-body correlations, ordinary nuclear structure, and concepts of
halos and Efimov states
A combined mean-field and three-body model tested on the O-nucleus
We combine few- and many-body degrees of freedom in a model applicable to
both bound and continuum states and adaptable to different subfields of
physics. We formulate a self-consistent three-body model for a core-nucleus
surrounded by two valence nucleons. We treat the core in the mean-field
approximation and use the same effective Skyrme interaction between both core
and valence nucleons. We apply the model to O where we reproduce the
known experimental data as well as phenomenological models with more
parameters. The decay of the ground state is found to proceed directly into the
continuum without effect of the virtual sequential decay through the well
reproduced -resonance of O.Comment: 5 pages, 5 figures, under revie
Combining few-body cluster structures with many-body mean-field methods
Nuclear cluster physics implicitly assumes a distinction between groups of
degrees-of-freedom, that is the (frozen) intrinsic and (explicitly treated)
relative cluster motion. We formulate a realistic and practical method to
describe the coupled motion of these two sets of degrees-of-freedom. We derive
a coupled set of differential equations for the system using the
phenomenologically adjusted effective in-medium Skyrme type of nucleon-nucleon
interaction. We select a two-nucleon plus core system where the mean-field
approximation corresponding to the Skyrme interaction is used for the core. A
hyperspherical adiabatic expansion of the Faddeev equations is used for the
relative cluster motion. We shall specifically compare both the structure and
the decay mechanism found from the traditional three-body calculations with the
result using the new boundary condition provided by the full microscopic
structure at small distance. The extended Hilbert space guaranties an improved
wave function compared to both mean-field and three-body solutions. We shall
investigate the structures and decay mechanism of C (C+n+n). In
conclusion, we have developed a method combining nuclear few- and many-body
techniques without losing the descriptive power of each approximation at
medium-to-large distances and small distances respectively. The coupled set of
equations are solved self-consistently, and both structure and dynamic
evolution are studied.Comment: 4 pages, 3 figures, conference proceedings, publishe
Two-proton capture on the Se nucleus with a new self-consistent cluster model
We investigate the two-proton capture reaction of the prominent rapid proton
capture waiting point nucleus, Se, that produces the borromean nucleus
Kr (Se). We apply a recently formulated general model where
the core nucleus, Se, is treated in the mean-field approximation and the
three-body problem of the two valence protons and the core is solved exactly.
The same Skyrme interaction is used to find core-nucleon and core
valence-proton interactions. We calculate electromagnetic two-proton
dissociation and capture cross sections, and derive the temperature dependent
capture rates. We vary the unknown resonance energy without changing any
of the structures computed self-consistently for both core and valence
particles. We find rates increasing quickly with temperature below ~GK
after which we find rates varying by less than a factor of two independent of
resonance energy. The capture mechanism is sequential through the
proton-core resonance, but the continuum background contributes
significantly.Comment: 7 pages, 4 figure
Density of states determined from Monte Carlo simulations
We describe method for calculating the density of states by combining several
canonical monte carlo runs. We discuss how critical properties reveal
themselves in and demonstrate this by applying the method several
different phase transitions. We also demonstrate how this can used to calculate
the conformal charge, where the dominating numerical method has traditionally
been transfer matrix.Comment: Major revision of paper, several references added throughout. Current
version accepted for publication in Phys. Rev.
Hausdorff dimension of critical fluctuations in abelian gauge theories
The geometric properties of the critical fluctuations in abelian gauge
theories such as the Ginzburg-Landau model are analyzed in zero background
field. Using a dual description, we obtain scaling relations between exponents
of geometric and thermodynamic nature. In particular we connect the anomalous
scaling dimension of the dual matter field to the Hausdorff dimension
of the critical fluctuations, {\it which are fractal objects}. The
connection between the values of and , and the possibility of
having a thermodynamic transition in finite background field, is discussed.Comment: Accepted for publication in PR
Manifestation of quantum chaos on scattering techniques: application to low-energy and photo-electron diffraction intensities
Intensities of LEED and PED are analyzed from a statistical point of view.
The probability distribution is compared with a Porter-Thomas law,
characteristic of a chaotic quantum system. The agreement obtained is
understood in terms of analogies between simple models and Berry's conjecture
for a typical wavefunction of a chaotic system. The consequences of this
behaviour on surface structural analysis are qualitatively discussed by looking
at the behaviour of standard correlation factors.Comment: 5 pages, 4 postscript figures, Latex, APS,
http://www.icmm.csic.es/Pandres/pedro.ht
Multiplicity Distributions and Rapidity Gaps
I examine the phenomenology of particle multiplicity distributions, with
special emphasis on the low multiplicities that are a background in the study
of rapidity gaps. In particular, I analyze the multiplicity distribution in a
rapidity interval between two jets, using the HERWIG QCD simulation with some
necessary modifications. The distribution is not of the negative binomial form,
and displays an anomalous enhancement at zero multiplicity. Some useful
mathematical tools for working with multiplicity distributions are presented.
It is demonstrated that ignoring particles with pt<0.2 has theoretical
advantages, in addition to being convenient experimentally.Comment: 24 pages, LaTeX, MSUHEP/94071
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