1,068 research outputs found
Monte Carlo simulation of uncoupled continuous-time random walks yielding a stochastic solution of the space-time fractional diffusion equation
We present a numerical method for the Monte Carlo simulation of uncoupled
continuous-time random walks with a Levy alpha-stable distribution of jumps in
space and a Mittag-Leffler distribution of waiting times, and apply it to the
stochastic solution of the Cauchy problem for a partial differential equation
with fractional derivatives both in space and in time. The one-parameter
Mittag-Leffler function is the natural survival probability leading to
time-fractional diffusion equations. Transformation methods for Mittag-Leffler
random variables were found later than the well-known transformation method by
Chambers, Mallows, and Stuck for Levy alpha-stable random variables and so far
have not received as much attention; nor have they been used together with the
latter in spite of their mathematical relationship due to the geometric
stability of the Mittag-Leffler distribution. Combining the two methods, we
obtain an accurate approximation of space- and time-fractional diffusion
processes almost as easy and fast to compute as for standard diffusion
processes.Comment: 7 pages, 5 figures, 1 table. Presented at the Conference on Computing
in Economics and Finance in Montreal, 14-16 June 2007; at the conference
"Modelling anomalous diffusion and relaxation" in Jerusalem, 23-28 March
2008; et
Evaluation of contamination for extraneous materials in “sun meat” sold in the “houses of the north” in the municipality of Diadema (SP, Brazil)
The “sun meat” is a handmade product, combining surface techniques of salting and dehydration, commonly used by people from the North and Northeast regions of Brazil. The sun meat cooking process lacks in sophisticated technology and official standards of procedure and quality. Thus, production, sale and distribution under unsatisfactory sanitary conditions are risky to the consumers’ health. This paper is aimed at evaluating the sanitary conditions of “sun meat” sold at the “houses of the north” for the presence of extraneous materials. Analysis of 44 samples of “sun meat” from 22 “houses of the north” located in “Diadema District” (SP, Brazil) had been carried out. The product conditions of exposure and sale in the “houses of the north” were evaluated and provided additional information to assess the degree of food safety of this product. In 44 samples were found various types of extraneous materials such as whole insects and debris, larvae, exuvia, mites, rodent hairs, bird feather pieces, fungi, and sharp objects. Mechanical vectors were observed at 11 locations of the sale area. The results indicated that 90.9% of the “sun meat” has unsatisfactory sanitary conditions, caused by the presence of physical hazards of contamination. These results, associated with conditions found in the “houses of the north”, indicated that these products can put the health of consumers at risk. Keywords: Sun meat, Sanitary Conditions, Food security
Hard X-ray spectroscopy of the itinerant magnets FeSb (Na, K, Ca, Sr, Ba)
Ordered states in itinerant magnets may be related to magnetic moments
displaying some weak local moment characteristics, as in intermetallic
compounds hosting transition metal coordination complexes. In this paper, we
report on the Fe -edge X-ray absorption spectroscopy (XAS) of the itinerant
magnets FeSb ( Na, K, Ca, Sr, Ba), aiming at exploring the
electronic and structural properties of the octahedral building block formed by
Fe and the Sb ligands. We find evidence for strong hybridization between the Fe
and Sb states at the Fermi level, giving experimental support to
previous electronic structure calculations of the FeSb
skutterudites. The electronic states derived from Fe 3 Sb mixing are
shown to be either more occupied and/or less localized in the cases of the
magnetically ordered systems, for which Na or K, connecting the local Fe
electronic structure to the itinerant magnetic properties. Moreover, the
analysis of the extended region of the XAS spectra (EXAFS) suggests that bond
disorder may be a more relevant parameter to explain the suppression of the
ferromagnetic ordered state in CaFeSb than the decrease of the
density of states.Comment: 7 pages, 5 figures, submitte
Magnetic Fields at First Order Phase Transition: A Threat to Electroweak Baryogenesis
The generation of the observed baryon asymmetry may have taken place during
the electroweak phase transition, thus involving physics testable at LHC, a
scenario dubbed electroweak baryogenesis. In this paper we point out that the
magnetic field which is produced in the bubbles of a first order phase
transition endangers the baryon asymmetry produced in the bubble walls. The
reason being that the produced magnetic field couples to the sphaleron magnetic
moment and lowers the sphaleron energy; this strengthens the sphaleron
transitions inside the bubbles and triggers a more effective wash out of the
baryon asymmetry. We apply this scenario to the Minimal Supersymmetric
extension of the Standard Model (MSSM) where, in the absence of a magnetic
field, successful electroweak baryogenesis requires the lightest CP-even Higgs
and the right-handed stop masses to be lighter than about 127 GeV and 120 GeV,
respectively. We show that even for moderate values of the magnetic field, the
Higgs mass required to preserve the baryon asymmetry is below the present
experimental bound. As a consequence electroweak baryogenesis within the MSSM
should be confronted on the one hand to future measurements at the LHC on the
Higgs and the right-handed stop masses, and on the other hand to more precise
calculations of the magnetic field produced at the electroweak phase
transition.Comment: 16 pages, 4 figures. Minor corrections and references added to match
published versio
Liquid crystal director fluctuations and surface anchoring by molecular simulation
We propose a simple and reliable method to measure the liquid crystal surface
anchoring strength by molecular simulation. The method is based on the
measurement of the long-range fluctuation modes of the director in confined
geometry. As an example, molecular simulations of a liquid crystal in slab
geometry between parallel walls with homeotropic anchoring have been carried
out using the Monte Carlo technique. By studying different slab thicknesses, we
are able to calculate separately the position of the elastic boundary
condition, and the extrapolation length
Discovery of a Luminous Quasar in the Nearby Universe
In the course of the Pico dos Dias survey (PDS), we identified the stellar
like object PDS456 at coordinates alpha = 17h 28m 19.796s, delta = -14deg 15'
55.87'' (epoch 2000), with a relatively nearby (z = 0.184) and bright (B =
14.69) quasar. Its position at Galactic coordinates l_II = 10.4deg, b_II =
+11.2deg, near the bulge of the Galaxy, may explain why it was not detected
before. The optical spectrum of PDS456 is typical of a luminous quasar, showing
a broad (FWHM ~ 4000 km/s) H_\beta line, very intense FeII lines and a weak
[OIII]\lambda5007 line. PDS456 is associated to the infrared source IRAS
17254-1413 with a 60 \mum infrared luminosity L_{60} = 3.8 x 10^{45} erg/s. The
relatively flat slopes in the infrared (\alpha(25,60) = -0.33 and \alpha(12,25)
= -0.78) and a flat power index in the optical (F_{\nu} \propto \nu^{-0.72})
may indicate a low dust content. A good match between the position of PDS456
and the position of the X-ray source RXS J172819.3-141600 implies an X-ray
luminosity L_x = 2.8 x 10^{44} erg/s. The good correlation between the strength
of the emission lines in the optical and the X-ray luminosity, as well as the
steep optical to X-ray index estimated (\alpha_{ox} = -1.64) suggest that
PDS456 is radio quiet. A radio survey previously performed in this region
yields an upper limit for radio power at ~ 5 GHz of ~ 2.6 x 10^{30} erg/s/Hz.
We estimate the Galactic reddening in this line-of-sight to be A_B \simeq 2.0,
implying an absolute magnitude M_B = -26.7 (using H_0 = 75 km s^{-1} Mpc^{-1}
and q_0 = 0). In the optical, PDS456 is therefore 1.3 times more luminous than
3C 273 and the most luminous quasar in the nearby (z \leq 0.3) Universe.Comment: 12 pages, LaTeX (aasms4.sty) + 3 figures; accepted for publication in
the Astrophysical Journal Letter
Prognostic utility of sestamibi lung uptake does not require adjustment for stress-related variables: A retrospective cohort study
BACKGROUND: Increased (99m)Tc-sestamibi stress lung-to-heart ratio (sLHR) has been shown to predict cardiac outcomes similar to pulmonary uptake of thallium. Peak heart rate and use of pharmacologic stress affect the interpretation of lung thallium uptake. The current study was performed to determine whether (99m)Tc-sestamibi sLHR measurements are affected by stress-related variables, and whether this in turn affects prognostic utility. METHODS: sLHR was determined in 718 patients undergoing (99m)Tc-sestamibi SPECT stress imaging. sLHR was assessed in relation to demographics, hemodynamic variables and outcomes (mean follow up 5.6 ± 1.1 years). RESULTS: Mean sLHR was slightly greater in males than in females (P < 0.01) and also showed a weak negative correlation with age (P < 0.01) and systolic blood pressure (P < 0.01), but was unrelated to stress method or heart rate at the time of injection. In patients undergoing treadmill exercise, sLHR was also positively correlated with peak workload (P < 0.05) but inversely with double product (P < 0.05). The combined explanatory effect of sex, age and hemodynamic variables on sLHR was less than 10%. The risk of acute myocardial infarction (AMI) or death increased by a factor of 1.7–1.8 for each SD increase in unadjusted sLHR, and was unaffected by adjustment for sex, age and hemodynamic variables (hazard ratios 1.6–1.7). The area under the ROC curve for the unadjusted sLHR was 0.65 (95% CI 0.59–0.71, P < 0.0001) and was unchanged for the adjusted sLHR (0.65, 95% CI 0.61–0.72, P < 0.0001). CONCLUSION: Stress-related variables have only a weak effect on measured sLHR. Unadjusted and adjusted sLHR provide equivalent prognostic information for prediction of AMI or death
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