5,058 research outputs found
Operator product expansion and quark condensate from Lattice QCD in coordinate space
We present a Lattice QCD determination of the chiral quark condensate based
on a new method. We extract the quark condensate from the operator product
expansion of the quark propagator at short euclidean distances, where it
represents the leading contribution in the chiral limit. From this study we
obtain ^ms(2 GeV)=-(265+-5+-22 MeV)^3$, in good agreement with
determinations of this quantity based on different approaches. The simulation
is performed by using the O(a)-improved Wilson action at beta=6.45 on a volume
32^3\times70 in the quenched approximation
Renormalization Constants of Quark Operators for the Non-Perturbatively Improved Wilson Action
We present the results of an extensive lattice calculation of the
renormalization constants of bilinear and four-quark operators for the
non-perturbatively O(a)-improved Wilson action. The results are obtained in the
quenched approximation at four values of the lattice coupling by using the
non-perturbative RI/MOM renormalization method. Several sources of systematic
uncertainties, including discretization errors and final volume effects, are
examined. The contribution of the Goldstone pole, which in some cases may
affect the extrapolation of the renormalization constants to the chiral limit,
is non-perturbatively subtracted. The scale independent renormalization
constants of bilinear quark operators have been also computed by using the
lattice chiral Ward identities approach and compared with those obtained with
the RI-MOM method. For those renormalization constants the non-perturbative
estimates of which have been already presented in the literature we find an
agreement which is typically at the level of 1%.Comment: 36 pages, 13 figures. Minor changes in the text and in one figure.
Accepted for publication on JHE
A Computational Approach to Multistationarity of Power-Law Kinetic Systems
This paper presents a computational solution to determine if a chemical
reaction network endowed with power-law kinetics (PLK system) has the capacity
for multistationarity, i.e., whether there exist positive rate constants such
that the corresponding differential equations admit multiple positive steady
states within a stoichiometric class. The approach, which is called the
"Multistationarity Algorithm for PLK systems" (MSA), combines (i) the extension
of the "higher deficiency algorithm" of Ji and Feinberg for mass action to PLK
systems with reactant-determined interactions, and (ii) a method that
transforms any PLK system to a dynamically equivalent one with
reactant-determined interactions. Using this algorithm, we obtain two new
results: the monostationarity of a popular model of anaerobic yeast
fermentation pathway, and the multistationarity of a global carbon cycle model
with climate engineering, both in the generalized mass action format of
biochemical systems theory. We also provide examples of the broader scope of
our approach for deficiency one PLK systems in comparison to the extension of
Feinberg's "deficiency one algorithm" to such systems
MetodologĂa para la restauraciĂłn y puesta en marcha de una máquina fresadora CNC
The Computerized Numerical Control (CNC) is a technology that allows, through the use of a computer, to control and monitor the movements of a machine tool. The Superior Technological Institute of San AndrĂ©s Tuxtla (ITSSAT) has a CNC milling machine tool with European technology acquired by the state government in 2003 for engineering student practices. The machine was obsolete and out of operation, a situation that obligates not to use the CNC in practice for a while since the industries that manufacture them use licensed software and electronic cards that have a high cost for repair. The purpose of this article is to present a methodology for the restoration and start-up of CNC machines without having to resort to the manufacturer. The results show the methodology whose main function is the communication of the CNC machine with the PC through the implementation of the alternate software Mach 3 and the electronic card LPT-Mach 3 achieving the operation of the machine and the milling of materials with academic and industrial purposes.El Control NumĂ©rico Computarizado (CNC) es una tecnologĂa que permite mediante el uso de una computadora controlar y monitorear los movimientos de una máquina herramienta. El Instituto TecnolĂłgico Superior de San AndrĂ©s Tuxtla (ITSSAT) cuenta con una máquina herramienta fresadora CNC con tecnologĂa europea adquirida por el gobierno del Estado en el 2003 para prácticas de los estudiantes de ingenierĂa. La máquina se encontraba obsoleta y sin funcionamiento, situaciĂłn que provoco no hacer uso de la CNC en las prácticas durante un tiempo, ya que las industrias que las fabrican utilizan software con licencia y tarjetas electrĂłnicas que tienen un costo elevado para su reparaciĂłn. El presente artĂculo tiene como finalidad dar a conocer una metodologĂa para la restauraciĂłn y puesta en marcha de la máquina fresadora CNC sin necesidad de recurrir al fabricante. Los resultados muestran la metodologĂa cuya principal funciĂłn es la comunicaciĂłn de la máquina CNC con la PC mediante la implementaciĂłn del software alterno Mach 3 y la tarjeta electrĂłnica LPT-Mach 3 logrando el funcionamiento de la máquina y con ello el fresado de materiales con fines acadĂ©micos e industriales
The non-convex shape of (234) Barbara, the first Barbarian
Asteroid (234) Barbara is the prototype of a category of asteroids that has
been shown to be extremely rich in refractory inclusions, the oldest material
ever found in the Solar System. It exhibits several peculiar features, most
notably its polarimetric behavior. In recent years other objects sharing the
same property (collectively known as "Barbarians") have been discovered.
Interferometric observations in the mid-infrared with the ESO VLTI suggested
that (234) Barbara might have a bi-lobated shape or even a large companion
satellite. We use a large set of 57 optical lightcurves acquired between 1979
and 2014, together with the timings of two stellar occultations in 2009, to
determine the rotation period, spin-vector coordinates, and 3-D shape of (234)
Barbara, using two different shape reconstruction algorithms. By using the
lightcurves combined to the results obtained from stellar occultations, we are
able to show that the shape of (234) Barbara exhibits large concave areas.
Possible links of the shape to the polarimetric properties and the object
evolution are discussed. We also show that VLTI data can be modeled without the
presence of a satellite.Comment: 10 pages, 6 figure
Coastal high-frequency radars in the Mediterranean - Part 2: Applications in support of science priorities and societal needs
The Mediterranean Sea is a prominent climate-change hot spot, with many socioeconomically vital coastal areas being the most vulnerable targets for maritime safety, diverse met-ocean hazards and marine pollution. Providing an unprecedented spatial and temporal resolution at wide coastal areas, high-frequency radars (HFRs) have been steadily gaining recognition as an effective land-based remote sensing technology for continuous monitoring of the surface circulation, increasingly waves and occasionally winds. HFR measurements have boosted the thorough scientific knowledge of coastal processes, also fostering a broad range of applications, which has promoted their integration in coastal ocean observing systems worldwide, with more than half of the European sites located in the Mediterranean coastal areas. In this work, we present a review of existing HFR data multidisciplinary science-based applications in the Mediterranean Sea, primarily focused on meeting end-user and science-driven requirements, addressing regional challenges in three main topics: (i) maritime safety, (ii) extreme hazards and (iii) environmental transport process. Additionally, the HFR observing and monitoring regional capabilities in the Mediterranean coastal areas required to underpin the underlying science and the further development of applications are also analyzed. The outcome of this assessment has allowed us to provide a set of recommendations for future improvement prospects to maximize the contribution to extending science-based HFR products into societally relevant downstream services to support blue growth in the Mediterranean coastal areas, helping to meet the UN's Decade of Ocean Science for Sustainable Development and the EU's Green Deal goals
Making Galaxies in a Cosmological Context: The Need for Early Stellar Feedback
We introduce the Making Galaxies in a Cosmological Context (MaGICC) program
of smoothed particle hydrodynamics (SPH) simulations. We describe a parameter
study of galaxy formation simulations of an L* galaxy that uses early stellar
feedback combined with supernova feedback to match the stellar mass--halo mass
relationship. While supernova feedback alone can reduce star formation enough
to match the stellar mass--halo mass relationship, the galaxy forms too many
stars before z=2 to match the evolution seen using abundance matching. Our
early stellar feedback is purely thermal and thus operates like a UV ionization
source as well as providing some additional pressure from the radiation of
massive, young stars. The early feedback heats gas to >10^6 K before cooling to
10^4 K. The pressure from this hot gas creates a more extended disk and
prevents more star formation prior to z=1 than supernovae feedback alone. The
resulting disk galaxy has a flat rotation curve, an exponential surface
brightness profile, and matches a wide range of disk scaling relationships. The
disk forms from the inside-out with an increasing exponential scale length as
the galaxy evolves. Overall, early stellar feedback helps to simulate galaxies
that match observational results at low and high redshifts.Comment: 13 pages, 14 figures, accepted MNRAS, movies at
http://www.mpia.de/~stinson/magic
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