16,100 research outputs found
Lattice QCD Production on Commodity Clusters at Fermilab
We describe the construction and results to date of Fermilab's three
Myrinet-networked lattice QCD production clusters (an 80-node dual Pentium III
cluster, a 48-node dual Xeon cluster, and a 128-node dual Xeon cluster). We
examine a number of aspects of performance of the MILC lattice QCD code running
on these clusters.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 6 pages, LaTeX, 8 eps figures. PSN
TUIT00
Objective multiscale analysis of random heterogeneous materials
The multiscale framework presented in [1, 2] is assessed in this contribution for a study of random heterogeneous materials. Results are compared to direct numerical simulations (DNS) and the sensitivity to user-defined parameters such as the domain decomposition type and initial coarse scale resolution is reported. The parallel performance of the implementation is studied for different domain decompositions
Phase diagrams of charged colloidal rods: can a uniaxial charge distribution break chiral symmetry?
We construct phase diagrams for charged rodlike colloids within the
second-virial approximation as a function of rod concentration, salt
concentration, and colloidal charge. Besides the expected isotropic-nematic
transition, we also find parameter regimes with a coexistence between a nematic
and a second, more highly aligned nematic phase including an
isotropic-nematic-nematic triple point and a nematic-nematic critical point,
which can all be explained in terms of the twisting effect. We compute the
Frank elastic constants to see if the twist elastic constant can become
negative, which would indicate the possibility of a cholesteric phase
spontaneously forming. Although the twisting effect reduces the twist elastic
constant, we find that it always remains positive. In addition, we find that
for finite aspect-ratio rods the twist elastic constant is also always
positive, such that there is no evidence of chiral symmetry breaking due to a
uniaxial charge distribution.Comment: Added a reference to Sec. 4 and extended discussions in Secs. 4 and
7, results unchange
Predicting the cosmological constant with the scale-factor cutoff measure
It is well known that anthropic selection from a landscape with a flat prior
distribution of cosmological constant Lambda gives a reasonable fit to
observation. However, a realistic model of the multiverse has a physical volume
that diverges with time, and the predicted distribution of Lambda depends on
how the spacetime volume is regulated. We study a simple model of the
multiverse with probabilities regulated by a scale-factor cutoff, and calculate
the resulting distribution, considering both positive and negative values of
Lambda. The results are in good agreement with observation. In particular, the
scale-factor cutoff strongly suppresses the probability for values of Lambda
that are more than about ten times the observed value. We also discuss several
qualitative features of the scale-factor cutoff, including aspects of the
distributions of the curvature parameter Omega and the primordial density
contrast Q.Comment: 16 pages, 6 figures, 2 appendice
The Hyperfine Splitting in Charmonium: Lattice Computations Using the Wilson and Clover Fermion Actions
We compute the hyperfine splitting on the lattice,
using both the Wilson and -improved (clover) actions for quenched quarks.
The computations are performed on a lattice at ,
using the same set of 18 gluon configurations for both fermion actions. We find
that the splitting is 1.83\err{13}{15} times larger with the clover action than
with the Wilson action, demonstrating the sensitivity of the spin-splitting to
the magnetic moment term which is present in the clover action. However, even
with the clover action the result is less than half of the physical
mass-splitting. We also compute the decay constants and
, both of which are considerably larger when computed using
the clover action than with the Wilson action. For example for the ratio
we find 0.32\err{1}{2} with the Wilson action
and with the clover action (the physical value is 0.44(2)).Comment: LaTeX file, 8 pages and two postscript figures. Southampton Preprint:
SHEP 91/92-27 Edinburgh Preprint: 92/51
Theory of continuum percolation II. Mean field theory
I use a previously introduced mapping between the continuum percolation model
and the Potts fluid to derive a mean field theory of continuum percolation
systems. This is done by introducing a new variational principle, the basis of
which has to be taken, for now, as heuristic. The critical exponents obtained
are , and , which are identical with the mean
field exponents of lattice percolation. The critical density in this
approximation is \rho_c = 1/\ve where \ve = \int d \x \, p(\x) \{ \exp [-
v(\x)/kT] - 1 \}. p(\x) is the binding probability of two particles
separated by \x and v(\x) is their interaction potential.Comment: 25 pages, Late
First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment
In this paper we investigate by means of first-principles density functional
theory calculations the (111) surface of the Ag-Cu alloy under varying
conditions of pressure of the surrounding oxygen atmosphere and temperature.
This alloy has been recently proposed as a catalyst with improved selectivity
for ethylene epoxidation with respect to pure silver, the catalyst commonly
used in industrial applications. Here we show that the presence of oxygen leads
to copper segregation to the surface. Considering the surface free energy as a
function of the surface composition, we construct the convex hull to
investigate the stability of various surface structures. By including the
dependence of the free surface energy on the oxygen chemical potential, we are
able compute the phase diagram of the alloy as a function of temperature,
pressure and surface composition. We find that, at temperature and pressure
typically used in ethylene epoxidation, a number of structures can be present
on the surface of the alloy, including clean Ag(111), thin layers of copper
oxide and thick oxide-like structures. These results are consistent with, and
help explain, recent experimental results.Comment: 10 pages, 6 figure
An X-ray and Optical Study of Matter Distribution in the Galaxy Cluster A 2319
A new analysis of velocity distribution, optical photometry and X-ray surface
brightness from ROSAT PSPC data of the galaxy cluster A 2319 is presented. The
temperature profile derived from ASCA data (Markevitch et al.,1996) is taken
into account. A method to check the hydrostatic model in the presence of a
temperature gradient is proposed. Consistency of the hydrostatic isothermal
model and the explanation of the "beta-discrepancy" are discussed. Galaxy and
gas density profiles of the main component A 2319A are derived, allowing for
the effect of the secondary component A 2319B. The inadequacy of a polytropic
model, which would produce a binding mass decrease with respect to the
isothermal beta-model, is discussed. A simple interpolation of the temperature
profile provides instead an increase of the binding mass and a lower baryon
fraction thus mitigating the "baryon catastrophe". Assuming as typical the
value f_b ~ 0.2, a comparison with the most recent estimate of Omega_b(nucl)
implies for the cosmological parameter Omega_o less than 0.4.Comment: 7 pages, 2 tables, 8 figure
Perfusion machines and hepatocellular carcinoma: a good match between a marginal organ and an advanced disease?
Abstract
Hepatocellular carcinoma (HCC) accounts for 90% of primary liver cancers, is the second leading cause of cancer-related deaths and the leading cause of death in patients with cirrhosis. Liver transplantation (LT) represents the ideal treatment for selected patients as it removes both the tumor and the underlying cirrhotic liver with 5-year survival rates higher than 70%. Unfortunately, due to tumor characteristics, patient co-morbidities or shortage of organs available for transplant, only 20% of patients can undergo curative treatment. Ex situ machine perfusion (MP) is a technology recently introduced that might potentially improve organ preservation, allow graft assessment and increase the pool of available organs. The purpose of this review is to provide an update on the current role of ex situ liver MP in liver transplantation for HCC patients
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