1,904 research outputs found
Continuum corrections to the level density and its dependence on excitation energy, n-p asymmetry, and deformation
In the independent-particle model, the nuclear level density is determined
from the neutron and proton single-particle level densities. The
single-particle level density for the positive-energy continuum levels is
important at high excitation energies for stable nuclei and at all excitation
energies for nuclei near the drip lines. This single-particle level density is
subdivided into compound-nucleus and gas components. Two methods were
considered for this subdivision. First in the subtraction method, the
single-particle level density is determined from the scattering phase shifts.
In the Gamov method, only the narrow Gamov states or resonances are included.
The level densities calculated with these two methods are similar, both can be
approximated by the backshifted Fermi-gas expression with level-density
parameters that are dependent on A, but with very little dependence on the
neutron or proton richness of the nucleus. However, a small decrease in the
level-density parameter was predicted for some nuclei very close to the drip
lines. The largest difference between the calculations using the two methods
was the deformation dependence on the level density. The Gamov method predicts
a very strong peaking of the level density at sphericity for high excitation
energies. This leads to a suppression of deformed configurations and,
consequently, the fission rate predicted by the statistical model is reduced in
the Gamov method.Comment: 18 pages 24 figure
Near-threshold boson pair production in the model of smeared-mass unstable particles
Near-threshold production of boson pairs is considered within the framework
of the model of unstable particles with smeared mass. We describe the principal
aspects of the model and consider the strategy of calculations including the
radiative corrections. The results of calculations are in good agreement with
LEP II data and Monte-Carlo simulations. Suggested approach significantly
simplifies calculations with respect to the standard perturbative one.Comment: 15 pages, 6 figures, minor corrections, references adde
High energy hadrons in EAS at mountain altitude
An extensive simulation has been carried out to estimate the physical
interpretation of dynamical factors such as , in terms of high
energy interaction features, concentrated in the present analysis on the
average transverse momentum. It appears that the large enhancement observed for
versus primary energy, suggesting in earliest analysis a significant
rise of with energy, is only the result of the limited resolution of the
detectors and remains in agreement with a wide range of models used in
simulations.Comment: 13 pages, 6 PostScript figures, LaTeX Subm. to JPhys
Single-Bottleneck Approximation for Driven Lattice Gases with Disorder and Open Boundary Conditions
We investigate the effects of disorder on driven lattice gases with open
boundaries using the totally asymmetric simple exclusion process as a
paradigmatic example. Disorder is realized by randomly distributed defect sites
with reduced hopping rate. In contrast to equilibrium, even macroscopic
quantities in disordered non-equilibrium systems depend sensitively on the
defect sample. We study the current as function of the entry and exit rates and
the realization of disorder and find that it is, in leading order, determined
by the longest stretch of consecutive defect sites (single-bottleneck
approximation, SBA). Using results from extreme value statistics the SBA allows
to study ensembles with fixed defect density which gives accurate results, e.g.
for the expectation value of the current. Corrections to SBA come from
effective interactions of bottlenecks close to the longest one. Defects close
to the boundaries can be described by effective boundary rates and lead to
shifts of the phase transitions. Finally it is shown that the SBA also works
for more complex models. As an example we discuss a model with internal states
that has been proposed to describe transport of the kinesin KIF1A.Comment: submitted to J. Stat. Mec
Isotope shift in the dielectronic recombination of three-electron ^{A}Nd^{57+}
Isotope shifts in dielectronic recombination spectra were studied for Li-like
^{A}Nd^{57+} ions with A=142 and A=150. From the displacement of resonance
positions energy shifts \delta E^{142,150}(2s-2p_1/2)= 40.2(3)(6) meV
(stat)(sys)) and \delta E^{142,150}(2s-2p_3/2) = 42.3(12)(20) meV of 2s-2p_j
transitions were deduced. An evaluation of these values within a full QED
treatment yields a change in the mean-square charge radius of ^{142,150}\delta
= -1.36(1)(3) fm^2. The approach is conceptually new and combines the
advantage of a simple atomic structure with high sensitivity to nuclear size.Comment: 10 pages, 3 figures, accepted for publication in Physical Review
Letter
Convergence and Gauge Dependence Properties of the Resummed One-loop Quark-Quark Scattering Amplitude in Perturbative QCD
The one-loop QCD effective charge for quark-quark scattering
is derived by diagrammatic resummation of the one-loop amplitude using an
arbitary covariant gauge. Except for the particular choice of gauge parameter
, is found to {\it increase} with increasing
physical scale, , as or . For ,
decreases with increasing and satisfies a renormalisation group equation.
Also, except for the case , convergence radii of geometric series
are found to impose upper limits on .Comment: 28 pages, 5 tables, 5 figures. v3 The one-loop amplitudes in Section
2 are recalculated using dimensional regularisation, and several errors in
the on-shell calculation of Reference[1] are pointed out. v4 one figure
removed one added. Three tables and new text in Section 5 added. Published
versio
MOST discovers a multimode delta Scuti star in a triple system: HD 61199
A field star, HD 61199 (V ~ 8), simultaneously observed with Procyon by the
MOST (Microvariability & Oscillations of STars) satellite in continuous runs of
34, 17, and 34 days in 2004, 2005, and 2007, was found to pulsate in 11
frequencies in the delta Scuti range with amplitudes from 1.7 down to 0.09
mmag. The photometry also showed variations with a period of about four days.
To investigate the nature of the longer period, 45 days of time-resolved
spectroscopy was obtained at the Thueringer Landessternwarte Tautenburg in
2004. The radial velocity measurements indicate that HD 61199 is a triple
system. A delta Scuti pulsator with a rich eigenspectrum in a multiple system
is promising for asteroseismology. Our objectives were to identify which of the
stars in the system is the delta Scuti variable and to obtain the orbital
elements of the system and the fundamental parameters of the individual
components, which are constrained by the pulsation frequencies of the delta
Scuti star. Classical Fourier techniques and least-squares multi-sinusoidal
fits were applied to the MOST photometry to identify the pulsation frequencies.
The groundbased spectroscopy was analysed with least-squares-deconvolution
(LSD) techniques, and the orbital elements derived with the KOREL and ORBITX
routines. Asteroseismic models were also generated. The photometric and
spectroscopic data are compatible with a triple system consisting of a close
binary with an orbital period of 3.57 days and a delta Scuti companion (HD
61199,A) as the most luminous component. The delta Scuti star is a rapid
rotator with about vsin i = 130 km/s and an upper mass limit of about 2.1 Msun.
For the close binary components, we find they are of nearly equal mass, with
lower mass limits of about 0.7 Msun.Comment: 11 pages, 14 figures, accepted by A&
Consistent Anisotropic Repulsions for Simple Molecules
We extract atom-atom potentials from the effective spherical potentials that
suc cessfully model Hugoniot experiments on molecular fluids, e.g., and
. In the case of the resulting potentials compare very well with the
atom-atom potentials used in studies of solid-state propertie s, while for
they are considerably softer at short distances. Ground state (T=0K) and
room temperatu re calculations performed with the new potential resolve
the previous discrepancy between experimental and theoretical results.Comment: RevTeX, 5 figure
Quantum Mechanical Aspects of Cell Microtubules: Science Fiction or Realistic Possibility?
Recent experimental research with marine algae points towards quantum
entanglement at ambient temperature, with correlations between essential
biological units separated by distances as long as 20 Angstr\"oms. The
associated decoherence times, due to environmental influences, are found to be
of order 400 fs. This prompted some authors to connect such findings with the
possibility of some kind of quantum computation taking place in these
biological entities: within the decoherence time scales, the cell "quantum
calculates" the optimal "path" along which energy and signal would be
transported more efficiently. Prompted by these experimental results, in this
talk I remind the audience of a related topic proposed several years ago in
connection with the possible r\^ole of quantum mechanics and/or field theory on
dissipation-free energy transfer in microtubules (MT), which constitute
fundamental cell substructures. Quantum entanglement between tubulin dimers was
argued to be possible, provided there exists sufficient isolation from other
environmental cell effects. The model was based on certain ferroelectric
aspects of MT. In the talk I review the model and the associated experimental
tests so far and discuss future directions, especially in view of the algae
photo-experiments.Comment: 31 pages latex, 11 pdf figures, uses special macros, Invited Plenary
Talk at DICE2010, Castello Pasquini, Castiglioncello (Italy), September 13-18
201
Fermi-liquid instabilities at magnetic quantum phase transitions
This review discusses instabilities of the Fermi-liquid state of conduction
electrons in metals with particular emphasis on magnetic quantum critical
points. Both the existing theoretical concepts and experimental data on
selected materials are presented; with the aim of assessing the validity of
presently available theory. After briefly recalling the fundamentals of
Fermi-liquid theory, the local Fermi-liquid state in quantum impurity models
and their lattice versions is described. Next, the scaling concepts applicable
to quantum phase transitions are presented. The Hertz-Millis-Moriya theory of
quantum phase transitions is described in detail. The breakdown of the latter
is analyzed in several examples. In the final part experimental data on
heavy-fermion materials and transition-metal alloys are reviewed and confronted
with existing theory.Comment: 62 pages, 29 figs, review article for Rev. Mod. Phys; (v2) discussion
extended, refs added; (v3) shortened; final version as publishe
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