4,090 research outputs found
Gauged WZW models for space-time groups and gravitational actions
In this paper we investigate gauged Wess-Zumino-Witten models for space-time
groups as gravitational theories, following the trend of recent work by
Anabalon, Willison and Zanelli. We discuss the field equations in any dimension
and study in detail the simplest case of two space-time dimensions and gauge
group SO(2,1). For this model we study black hole solutions and we calculate
their mass and entropy which resulted in a null value for both.Comment: 26 pages, no figure
Employing ternary fission of Pu as a probe of very neutron rich matter
Detailed assessments of the ability of recent theoretical approaches to
modeling existing experimental data for ternary fission confirm earlier
indications that the dominant mode of cluster formation in ternary fission is
clusterization in very neutron rich, very low density, essentially chemically
equilibrated, nucleonic matter. An extended study and comparison of these
approaches applied to ternary fission yields in the thermal neutron induced
reaction Pu(,f) has been undertaken to refine the
characterization of the source matter. The resonance gas approximation has been
improved taking in-medium effects on the binding energies into account. A
temperature of 1.29 MeV, density of nucleons/fm and
proton fraction = 0.035 are found to provide a good representation of
yields of the ternary emitted light particles and clusters. In particular,
results for and 2 isotopes are presented. Isotopes with larger are
discussed, and the roles of medium and continuum effects, even at very low
density are illustrated.Comment: 19 pages, 5 figure
A mechanistic-empirical based overlay design method for reflective cracking
This paper describes a new and innovative mechanistically based pavement overlay design method that considers the most predominant type of overlay distress observed in the field: Reflective cracking above old cracks in the underlying pavement surface. Both dense-graded hot mix asphalt and gap-graded asphalt rubber (wet process) mixes were studied, in the laboratory and in the field, to derive the necessary mechanistic relationships
and statistically based equations. The models proposed are based on a finite element model
that closely approximates actual field phenomena. Many field test sections, in Arizona, California and Portugal, were studied during the course of the research.
Other HMA mixes used for overlays may also be calibrated and used through the proposed
method. However, the relevant mix properties of any additional materials or environmental
zones must first be determined. The two mix types studied are mainly used in the desert
southwest region of Arizona and California. The overlay design program is available from
the Rubber Pavements Association or Arizona Department of Transportation in the form of an Excel spreadsheet with an easy-to-use visual basic computer program (macro)
Fish assemblages across the Mediterranean Sea and the effects of protection from fishing = I Popolamenti ittici nel Mediterraneo e gli effetti della protezione dall’impatto della pesca
Several studies have assessed the effectiveness of individual marine protected areas (MPAs) in protecting fish assemblages, but regional assessments of multiple parks are scarce. Here fish surveys using visual census were done in marine parks and fished areas at 31 locations across
the Mediterranean Sea. Fish species richness, diversity and biomass (especially of top predators) were higher in MPAs compared to fished areas, and community structure differed significantly between MPAs and fished areas. Results suggest that MPAs are generally effective means to protect and recover fish populations and assemblages
First Simultaneous Optical and EUV Observations of the Quasi-Coherent Oscillations of SS Cygni
Using EUV photometry obtained with the Extreme Ultraviolet Explorer (EUVE)
satellite and UBVR optical photometry obtained with the 2.7-m telescope at
McDonald Observatory, we have detected quasi-coherent oscillations (so-called
``dwarf nova oscillations'') in the EUV and optical flux of the dwarf nova SS
Cygni during its 1996 October outburst. There are two new results from these
observations. First, we have for the first time observed ``frequency
doubling:'' during the rising branch of the outburst, the period of the EUV
oscillation was observed to jump from 6.59 s to 2.91 s. Second, we have for the
first time observed quasi-coherent oscillations simultaneously in the optical
and EUV. We find that the period and phase of the oscillations are the same in
the two wavebands, finally confirming the long-held assumption that the periods
of the optical and EUV/soft X-ray oscillations of dwarf novae are equal. The
UBV oscillations can be simply the Rayleigh-Jeans tail of the EUV oscillations
if the boundary layer temperature kT_bb <~ 15 eV and hence the luminosity L_bb
>~ 1.2e34 (d/75 pc)^2 erg/s (comparable to that of the accretion disk).
Otherwise, the lack of a phase delay between the EUV and optical oscillations
requires that the optical reprocessing site lies within the inner third of the
accretion disk. This is strikingly different from other cataclysmic variables,
where much or all of the disk contributes to the optical oscillations.Comment: 16 pages including 3 tables and 4 encapsulated postscript figures;
LaTeX format, uses aastex.cls; accepted on 2001 August 2 for publication in
The Astrophysical Journa
Quark-hadron phase transition in a neutron star under strong magnetic fields
We study the effect of a strong magnetic field on the properties of neutron
stars with a quark-hadron phase transition. It is shown that the magnetic field
prevents the appearance of a quark phase, enhances the leptonic fraction,
decreases the baryonic density extension of the mixed phase and stiffens the
total equation of state, including both the stellar matter and the magnetic
field contributions. Two parametrisations of a density dependent static
magnetic field, increasing, respectively, fast and slowly with the density and
reaching G in the center of the star, are considered. The
compact stars with strong magnetic fields have maximum mass configurations with
larger masses and radius and smaller quark fractions. The parametrisation of
the magnetic field with density has a strong influence on the star properties.Comment: 15 pages, 6 figures, 8 tables, accepted for publication in J. Phys.
An Analysis of the Decay with Predictions from Heavy Quark and Chiral Symmetry
This paper considers the implications of the heavy quark and chiral
symmetries for the semi-leptonic decay . The general kinematic analysis for decays of the form {\sl
pseudoscalar meson vector meson pseudoscalar meson lepton
anti-lepton} is presented. This formalism is applied to the above exclusive
decay which allows the differential decay rate to be expressed in a form that
is ideally suited for the experimental determination of the different form
factors for the process through angular distribution measurements. Heavy quark
and chiral symmetry predictions for the form factors are presented, and the
differential decay rate is calculated in the kinematic region where chiral
perturbation theory is valid.Comment: 15 pages, uses jytex.tex and tables.tex; 3 figures not included but
available on reques
The Nature of the Secondary Star in the Black Hole X-Ray Transient V616 Mon (=A0620-00)
We have used NIRSPEC on Keck II to obtain -band spectroscopy of the low
mass X-ray binary V616 Mon (= A062000). V616 Mon is the proto-typical soft
x-ray transient containing a black hole primary. As such it is important to
constrain the masses of the binary components. The modeling of the infrared
observations of ellipsoidal variations in this system lead to a derived mass of
11.0 M_{\sun} for the black hole. The validity of this derivation has been
called into question due to the possiblity that the secondary star's spectral
energy distribution is contaminated by accretion disk emission (acting to
dilute the variations). Our new -band spectrum of V616 Mon reveals a
late-type K dwarf secondary star, but one that has very weak CO
absorption features. Comparison of V616 Mon with SS Cyg leads us to estimate
that the accretion disk supplies only a small amount of -band flux, and the
ellipsoidal variations are not seriously contaminated. If true, the derived
orbital inclination of V616 Mon is not greatly altered, and the mass of the
black hole remains large. A preliminary stellar atmosphere model for the
-band spectrum of V616 Mon reveals that the carbon abundance is
approximately 50% of the solar value. We conclude that the secondary star in
V616 Mon has either suffered serious contamination from the accretion of
supernova ejecta that created the black hole primary, or it is the stripped
remains of a formerly more massive secondary star, one in which the CNO cycle
had been active.Comment: 20 pages, 5 figure
Comment on evidence for new interference phenomena in the decay D+ -> K- pi+ mu+ nu
The experimental determination of low energy pi K scattering phase shifts
would assist in determining scattering lengths as well as low energy constants
of chiral perturbation theory for which sum rules have been constructed. The
FOCUS collaboration has presented evidence for interference pheomena from their
analysis of D_l4 decays based on decay amplitudes suitable for a cascade decay
D -> K* -> K pi. We point out that if the well-known full five body kinematics
are taken into account, pi K scattering phases may be extracted. We also point
out that other distributions considered in the context of K_l4 decays can be
applied to charm meson decays to provide constraints on violation of |Delta
I|=1/2 rule and T-violation.Comment: 9 pages, plain latex; version with minor changes compared to v1 on
lepton masses effects, sign error eliminated, clarifying remarks added, one
additional ref.; version to appear in Phys. Lett.
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