5,354 research outputs found
Transport properties of a meson gas
We present recent results on a systematic method to calculate transport
coefficients for a meson gas (in particular, we analyze a pion gas) at low
temperatures in the context of Chiral Perturbation Theory. Our method is based
on the study of Feynman diagrams with a power counting which takes into account
collisions in the plasma by means of a non-zero particle width. In this way, we
obtain results compatible with analysis of Kinetic Theory with just the leading
order diagram. We show the behavior with temperature of electrical and thermal
conductivities and shear and bulk viscosities, and we discuss the fundamental
role played by unitarity. We obtain that bulk viscosity is negligible against
shear viscosity near the chiral phase transition. Relations between the
different transport coefficients and bounds on them based on different
theoretical approximations are also discussed. We also comment on some
applications to heavy-ion collisions.Comment: 4 pages, 4 figures, IJMPE style. Contribution to the International
Workshop X Hadron Physics (2007), Florianopolis, Brazil. Accepted for
publication in IJMPE; 1 typo correcte
Bulk viscosity and the conformal anomaly in the pion gas
We calculate the bulk viscosity of the massive pion gas within Unitarized
Chiral Perturbation Theory. We obtain a low temperature peak arising from
explicit conformal breaking due to the pion mass and another peak near the
critical temperature, dominated by the conformal anomaly through gluon
condensate terms. The correlation between bulk viscosity and conformal breaking
supports a recent QCD proposal. We discuss the role of resonances, heavier
states and large- counting.Comment: Revised version accepted in Phys.Rev.Lett. 4 pages, 3 figure
Pion scattering poles and chiral symmetry restoration
Using unitarized Chiral Perturbation Theory methods, we perform a detailed
analysis of the scattering poles and behaviour
when medium effects such as temperature or density drive the system towards
Chiral Symmetry Restoration. In the analysis of real poles below threshold, we
show that it is crucial to extend properly the unitarized amplitudes so that
they match the perturbative Adler zeros. Our results do not show threshold
enhancement effects at finite temperature in the channel, which
remains as a pole of broad nature. We also implement T=0 finite density effects
related to chiral symmetry restoration, by varying the pole position with the
pion decay constant. Although this approach takes into account only a limited
class of contributions, we reproduce the expected finite density restoration
behaviour, which drives the poles towards the real axis, producing threshold
enhancement and bound states. We compare our results with several
model approaches and discuss the experimental consequences, both in
Relativistic Heavy Ion Collisions and in and
reactions in nuclei.Comment: 17 pages, 9 figures, final version to appear in Phys.Rev.D, added
comments and reference
Transport coefficients in Chiral Perturbation Theory
We present recent results on the calculation of transport coefficients for a
pion gas at zero chemical potential in Chiral Perturbation Theory using Linear
Response Theory. More precisely, we show the behavior of DC conductivity and
shear viscosity at low temperatures. To compute transport coefficients, the
standard power counting of ChPT has to be modified. The effects derived from
imposing unitarity are also analyzed. As physical applications in Relativistic
Heavy Ion Collisions, we show the relation of the DC conductivity to
soft-photon production and phenomenological effects related to a nonzero shear
viscosity. In addition, our values for the shear viscosity to entropy ratio
satisfy the KSS bound.Comment: 3 pages, 6 figures, EPJA style. Talk given at the QNP06 conference in
Madrid, and accepted for publication in EPJ
Partial mass concentration for fast-diffusions with non-local aggregation terms
We study well-posedness and long-time behaviour of aggregation-diffusion
equations of the form in the fast-diffusion
range, , and and regular enough. We develop a well-posedness
theory, first in the ball and then in , and characterise the
long-time asymptotics in the space for radial initial data. In the
radial setting and for the mass equation, viscosity solutions are used to prove
partial mass concentration asymptotically as , i.e. the limit as
is of the form with
and . Finally, we give instances of showing that partial mass concentration does happen in infinite time,
i.e.
Point defects on graphene on metals
Understanding the coupling of graphene with its local environment is critical
to be able to integrate it in tomorrow's electronic devices. Here we show how
the presence of a metallic substrate affects the properties of an atomically
tailored graphene layer. We have deliberately introduced single carbon
vacancies on a graphene monolayer grown on a Pt(111) surface and investigated
its impact in the electronic, structural and magnetic properties of the
graphene layer. Our low temperature scanning tunneling microscopy studies,
complemented by density functional theory, show the existence of a broad
electronic resonance above the Fermi energy associated with the vacancies.
Vacancy sites become reactive leading to an increase of the coupling between
the graphene layer and the metal substrate at these points; this gives rise to
a rapid decay of the localized state and the quenching of the magnetic moment
associated with carbon vacancies in free-standing graphene layers
Industrialización para la construcción de viviendas. Viviendas asequibles realizadas con prefabricados de hormigón
The need for housing has increased significantly during the last decades. This need is considerable at present in third world and developing countries, where the population has a rate of growth much larger than developed countries. Furthermore the population in the world is shifting to the cities. In the surrounding areas of third world and developing countries is where the need for housing is sometimes critical. In these places there is a need for affordable, housing, for low income groups. The Commission of Prefabrication of fib has been conscious for many years to this problem which becomes vital when natural disasters put in to many of the housing that is at present been built in these areas. Therefore a Group was created to develop a document about Affordable Housing that could be a help to the people for the use of prefabrication techniques in the construction of houses. This article resumes the works that have been developed by this group up to this moment.Hay una necesidad creciente para la construcción de viviendas, necesidad que se ha incrementado de forma exponencial en las últimas décadas. Esta necesidad es considerable en los países en vías de desarrollo, donde la población tiene una tasa de crecimiento elevada, mucho mayor que en los países desarrollados. Además, la población en el Mundo está desplazándose progresivamente hacia las ciudades. Es en los alrededores de las ciudades de países en desarrollo donde surge una necesidad de construir viviendas de coste controlado, asequibles, que se convierte en algunos casos en crítica. La Comisión de Prefabricación de la fib (International Federation for Structural Concrete, Fédération Internationale du Béton, Federación Internacional del Hormigón Estructural) ha sido consciente por muchos años de este problema y tomó la decisión de crear un grupo de trabajo para desarrollar un documento sobre1* viviendas prefabricadas de coste asequible. Este documento estará dedicado a servir de ayuda sobre las técnicas de la prefabricación para las personas que deseen construir este tipo de viviendas. El presente artículo hace un resumen del trabajo desarrollado por este grupo hasta el momento
Low-lying spectra in anharmonic three-body oscillators with a strong short-range repulsion
Three-body Schroedinger equation is studied in one dimension. Its two-body
interactions are assumed composed of the long-range attraction (dominated by
the L-th-power potential) in superposition with a short-range repulsion
(dominated by the (-K)-th-power core) plus further subdominant power-law
components if necessary. This unsolvable and non-separable generalization of
Calogero model (which is a separable and solvable exception at L = K = 2) is
presented in polar Jacobi coordinates. We derive a set of trigonometric
identities for the potentials which generalizes the well known K=2 identity of
Calogero to all integers. This enables us to write down the related partial
differential Schroedinger equation in an amazingly compact form. As a
consequence, we are able to show that all these models become separable and
solvable in the limit of strong repulsion.Comment: 18 pages plus 6 pages of appendices with new auxiliary identitie
Features of non-kinship foster care children with birth family contact in Andalusia
The aim of the presentation is to describe the personal features of non-kinship foster care children who have visits with their birth family in Málaga, Granada and Jaén (provinces of Andalusia, Spain). This study was funded by the research project Application of a psychoeducational intervention program to improve visits between foster children and their biological families (Reference EDU2016 77094-P). SPSS v.21.0 was used to carry out the descriptive and frequency analysis of socio-demographic information collected by the Child File Summary Form designed for this study. There are 212 non-kinship foster care children who have visits with their birth family. Their mean age is 8.09 years old (SD= 4.73). The more frequent types of foster care are both long-term and short-term placement (30.7%, respectively), followed by specialized long-term foster care (20.3%), urgency placement (10.8%) and specialized short-term foster care (7.5%). The period of time they have been into the Child Protection System is on average 4.08 years (SD= 3.88) and into the current placement is on average 2.53 (SD= 2.95). Thus, 44.8% of foster children were in residential care and 30.2% were in previous foster care. The latter could have been with the current foster care family. Also, 41.0% of foster children were placed at least with one sibling. It is important to know the features of foster children in order to apply the main principle of “the best interests of the child” to the whole decision-making process about the child’s protection and welfare. In our project, these decisions concern foster care placement and birth family contact to maintain and strengthen family affective bonds and child’s identity formation. Our findings contribute to develop: (a) initiatives to improve children’s well-being; (b) support and social resources required by families; and (c) socio-educative tools for the social workers.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
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