1,581 research outputs found
Conductance Distributions in Random Resistor Networks: Self Averaging and Disorder Lengths
The self averaging properties of conductance are explored in random
resistor networks with a broad distribution of bond strengths
P(g)\simg^{\mu-1}. Distributions of equivalent conductances are estimated
numerically on hierarchical lattices as a function of size and distribution
tail parameter . For networks above the percolation threshold, convergence
to a Gaussian basin is always the case, except in the limit --> 0. A {\it
disorder length} is identified beyond which the system is effectively
homogeneous. This length diverges as ( is the
regular percolation correlation length exponent) as -->0. This suggest
that exactly the same critical behavior can be induced by geometrical disorder
and bu strong bond disorder with the bond occupation probability .
Only lattices at the percolation threshold have renormalized probability
distribution in a {\it Levy-like} basin. At the threshold the disorder length
diverges at a vritical tail strength as , with
, a new exponent. Critical path analysis is used in a generalized
form to give form to give the macroscopic conductance for lattice above .Comment: 16 pages plain TeX file, 6 figures available upon
request.IBC-1603-01
On the Schroedinger Representation for a Scalar Field on Curved Spacetime
It is generally known that linear (free) field theories are one of the few
QFT that are exactly soluble. In the Schroedinger functional description of a
scalar field on flat Minkowski spacetime and for flat embeddings, it is known
that the usual Fock representation is described by a Gaussian measure. In this
paper, arbitrary globally hyperbolic space-times and embeddings of the Cauchy
surface are considered. The classical structures relevant for quantization are
used for constructing the Schroedinger representation in the general case. It
is shown that in this case, the measure is also Gaussian. Possible implications
for the program of canonical quantization of midisuperspace models are pointed
out.Comment: 11 pages, Revtex, no figure
Big bang nucleosynthesis with a varying fine structure constant and non-standard expansion rate
We calculate primordial abundances of light elements produced during big bang
nucleosynthesis when the fine structure constant and/or the cosmic expansion
rate take non-standard values. We compare them with the recent values of
observed D, He4 and Li7 abundances, which show slight inconsistency among
themselves in the standard big bang nucleosynthesis scenario. This
inconsistency is not solved by considering either a varying fine structure
constant or a non-standard expansion rate separately but solutions are found by
their simultaneous existence.Comment: 5 pages, 5 figure
First measurement of the 14N(p,gamma)15O cross section down to 70 keV
In stars with temperatures above 20*10^6 K, hydrogen burning is dominated by
the CNO cycle. Its rate is determined by the slowest process, the
14N(p,gamma)15O reaction. Deep underground in Italy's Gran Sasso laboratory, at
the LUNA 400 kV accelerator, the cross section of this reaction has been
measured at energies much lower than ever achieved before. Using a windowless
gas target and a 4pi BGO summing detector, direct cross section data has been
obtained down to 70 keV, reaching a value of 0.24 picobarn. The Gamow peak has
been covered by experimental data for several scenarios of stable and explosive
hydrogen burning. In addition, the strength of the 259 keV resonance has been
remeasured. The thermonuclear reaction rate has been calculated for
temperatures 90 - 300 *10^6 K, for the first time with negligible impact from
extrapolations
Symmetry breaking in numeric constraint problems
Symmetry-breaking constraints in the form of inequalities between variables have been proposed for a few kind of solution symmetries in numeric CSPs. We show that, for the variable symmetries among those, the proposed inequalities are but a specific case of a relaxation of the well-known LEX constraints extensively used for discrete CSPs. We discuss the merits of this relaxation and present experimental evidences of its practical interest.Postprint (authorâs final draft
First Stars. I. Evolution without mass loss
The first generation of stars was formed from primordial gas. Numerical
simulations suggest that the first stars were predominantly very massive, with
typical masses M > 100 Mo. These stars were responsible for the reionization of
the universe, the initial enrichment of the intergalactic medium with heavy
elements, and other cosmological consequences. In this work, we study the
structure of Zero Age Main Sequence stars for a wide mass and metallicity range
and the evolution of 100, 150, 200, 250 and 300 Mo galactic and pregalactic Pop
III very massive stars without mass loss, with metallicity Z=10E-6 and 10E-9,
respectively. Using a stellar evolution code, a system of 10 equations together
with boundary conditions are solved simultaneously. For the change of chemical
composition, which determines the evolution of a star, a diffusion treatment
for convection and semiconvection is used. A set of 30 nuclear reactions are
solved simultaneously with the stellar structure and evolution equations.
Several results on the main sequence, and during the hydrogen and helium
burning phases, are described. Low metallicity massive stars are hotter and
more compact and luminous than their metal enriched counterparts. Due to their
high temperatures, pregalactic stars activate sooner the triple alpha reaction
self-producing their own heavy elements. Both galactic and pregalactic stars
are radiation pressure dominated and evolve below the Eddington luminosity
limit with short lifetimes. The physical characteristics of the first stars
have an important influence in predictions of the ionizing photon yields from
the first luminous objects; also they develop large convective cores with
important helium core masses which are important for explosion calculations.Comment: 17 pages, 24 figures, 2 table
A new experiment for the determination of the 18F(p,alpha) reaction rate at nova temperatures
The 18F(p,alpha) reaction was recognized as one of the most important for
gamma ray astronomy in novae as it governs the early 511 keV emission. However,
its rate remains largely uncertain at nova temperatures. A direct measurement
of the cross section over the full range of nova energies is impossible because
of its vanishing value at low energy and of the short 18F lifetime. Therefore,
in order to better constrain this reaction rate, we have performed an indirect
experiment taking advantage of the availability of a high purity and intense
radioactive 18F beam at the Louvain La Neuve RIB facility. We present here the
first results of the data analysis and discuss the consequences.Comment: Contribution to the Classical Novae Explosions conference, Sitges,
Spain, 20-24 May 2002, 5 pages, 3 figure
Low energy measurement of the 7Be(p,gamma)8B cross section
We have measured the cross section of the 7Be(p,gamma)8B reaction for E_cm =
185.8 keV, 134.7 keV and 111.7 keV using a radioactive 7Be target (132 mCi).
Single and coincidence spectra of beta^+ and alpha particles from 8B and 8Be^*
decay, respectively, were measured using a large acceptance spectrometer. The
zero energy S factor inferred from these data is 18.5 +/- 2.4 eV b and a
weighted mean value of 18.8 +/- 1.7 eV b (theoretical uncertainty included) is
deduced when combining this value with our previous results at higher energies.Comment: Accepted for publication in Phys. Rev. Let
Feasibility of low energy radiative capture experiments at the LUNA underground accelerator facility
The LUNA (Laboratory Underground for Nuclear Astrophysics) facility has been
designed to study nuclear reactions of astrophysical interest. It is located
deep underground in the Gran Sasso National Laboratory, Italy. Two
electrostatic accelerators, with 50 and 400 kV maximum voltage, in combination
with solid and gas target setups allowed to measure the total cross sections of
the radiative capture reactions H(p,)3He and
N(p,)O within their relevant Gamow peaks. We report on
the gamma background in the Gran Sasso laboratory measured by germanium and
bismuth germanate detectors, with and without an incident proton beam. A method
to localize the sources of beam induced background using the Doppler shift of
emitted gamma rays is presented. The feasibility of radiative capture studies
at energies of astrophysical interest is discussed for several experimental
scenarios.Comment: Submitted to Eur. Phys. J.
Revision of the 15N(p,{\gamma})16O reaction rate and oxygen abundance in H-burning zones
The NO cycle takes place in the deepest layer of a H-burning core or shell,
when the temperature exceeds T {\simeq} 30 {\cdot} 106 K. The O depletion
observed in some globular cluster giant stars, always associated with a Na
enhancement, may be due to either a deep mixing during the RGB (red giant
branch) phase of the star or to the pollution of the primordial gas by an early
population of massive AGB (asymptotic giant branch) stars, whose chemical
composition was modified by the hot bottom burning. In both cases, the NO cycle
is responsible for the O depletion. The activation of this cycle depends on the
rate of the 15N(p,{\gamma})16O reaction. A precise evaluation of this reaction
rate at temperatures as low as experienced in H-burning zones in stellar
interiors is mandatory to understand the observed O abundances. We present a
new measurement of the 15N(p,{\gamma})16O reaction performed at LUNA covering
for the first time the center of mass energy range 70-370 keV, which
corresponds to stellar temperatures between 65 {\cdot} 106 K and 780 {\cdot}106
K. This range includes the 15N(p,{\gamma})16O Gamow-peak energy of explosive
H-burning taking place in the external layer of a nova and the one of the hot
bottom burning (HBB) nucleosynthesis occurring in massive AGB stars. With the
present data, we are also able to confirm the result of the previous R-matrix
extrapolation. In particular, in the temperature range of astrophysical
interest, the new rate is about a factor of 2 smaller than reported in the
widely adopted compilation of reaction rates (NACRE or CF88) and the
uncertainty is now reduced down to the 10% level.Comment: 6 pages, 5 figure
- âŠ