1,602 research outputs found
Electromagnetic fields and transport coefficients in a hot pion gas
We present recent results on finite temperature electromagnetic form factors
and the electrical conductivity in a pion gas. The standard Chiral Perturbation
Theory power counting needs to be modified for transport coefficients. We pay
special attention to unitarity and to possible applications for dilepton and
photon production.Comment: 4pp, 2 figures, talk given at "Strong and Electroweak Matter 2006",
BNL, May 200
Transport coefficients of a massive pion gas
We review or main results concerning the transport coefficients of a light
meson gas, in particular we focus on the case of a massive pion gas. Leading
order results according to the chiral power-counting are presented for the DC
electrical conductivity, thermal conductivity, shear viscosity, and bulk
viscosity. We also comment on the possible correlation between the bulk
viscosity and the trace anomaly in QCD, as well as the relation between
unitarity and a minimum of the quotient near the phase transition.Comment: Talk given at the 5th International Conference on Quarks and Nuclear
Physics (QNP09), Beijing, September 21-26, 200
Chiral symmetry and mesons in hot and dense matter: recent developments
We review recent results on properties of the meson gas relevant for Heavy
Ion Collision and Nuclear Matter experiments, within the framework of chiral
lagrangians. In particular, we describe the temperature and density evolution
of the and poles and its connection with chiral symmetry
restoration, as well as the chemical nonequilibrated phase and transport
coefficients.Comment: Proceedings of the "Chiral10 International Workshop on Chiral
Symmetry in Hadrons and Nuclei", Valencia, Spain, 21-24 june 2010. 9 pages, 5
figures. AIP Proceedings styl
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
The ratio of viscosity to entropy density in a pion gas satisfies the KSS holographic bound
We evaluate the ratio of shear viscosity to entropy density in a pion gas
employing the Uehling-Uehlenbeck equation and experimental phase-shifts
parameterized by means of the SU(2) Inverse Amplitude Method. We find that the
ratio for this monocomponent gas stays well above the KSS 1/(4 pi) bound. We
find similar results with other sets of phase shifts and conclude the bound is
nowhere violated.Comment: 2 page text, three figures. V2: short comment and graph added to
assert that a minimum of eta/s is not discarded from the hadron, low T side
in a heavy-ion collisio
Searching for Scuti-type pulsation and characterising northern pre-main-sequence field stars
A photometric variability study of a sample of northern field stars, which
previously classified as either PMS or Herbig Ae/Be objects, has been
undertaken with the purpose of detecting {\delta} Scuti-type pulsations.
Determination of physical parameters for these stars has also been carried out
to locate them on the HR diagram and check the instability strip for this type
of pulsators. Multichannel photomultiplier and CCD time series photometry in
the uvby Str\"omgren and BVI Johnson bands were obtained during four
consecutive years from 2007 to 2010. The light curves have been analysed, and a
variability criterion has been established. Among the objects classified as
variable stars, we have selected those which present periodicities above 4
d^(-1), which was established as the lowest limit for {\delta} Scuti-type
pulsations in this investigation. Finally, these variable stars have been
placed in a colour-magnitude diagram using the physical parameters derived with
the collected uvby{\beta} Str\"omgren-Crawford photometry. Five PMS {\delta}
Scuti- and three probable {\beta} Cephei-type stars have been detected. Two
additional PMS {\delta} Scuti stars are also confirmed in this work. Moreover,
three new {\delta} Scuti- and two {\gamma} Doradus-type stars have been
detected among the main-sequence objects used as comparison or check stars.Comment: Accepted for publication in Section 14. Catalogs and data of
Astronomy and Astrophysics. The official date of acceptance is 17/06/2014. 12
pages, 4 figures and 8 table
Atomistic simulations of magnetoelastic effects on sound velocity
In this work, we leverage atomistic spin-lattice simulations to examine how
magnetic interactions impact the propagation of sound waves through a
ferromagnetic material. To achieve this, we characterize the sound wave
velocity in BCC iron, a prototypical ferromagnetic material, using three
different approaches that are based on the oscillations of kinetic energy,
finite-displacement derived forces, and corrections to the elastic constants,
respectively. Successfully applying these methods within the spin-lattice
framework, we find good agreement with the Simon effect including high order
terms. In analogy to experiments, morphic coefficients associated with the
transverse and longitudinal waves propagating along the [001] direction are
extracted from fits to the fractional change in velocity data. The present
efforts represent an advancement in magnetoelastic modelling capabilities which
can expedite the design of future magneto-acoustic devices
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