58,827 research outputs found
Atomic Radiative Transitions in Thermo Field Dynamics
In this work we rederive the Lamb-Retherford energy shift for an atomic
electron in the presence of a thermal radiation. Using the Dalibard, Dupont-Roc
and Cohen-Tannoudji (DDC) formalism, where physical observables are expressed
as convolutions of suitable statistical functions, we construct the
electromagnetic field propagator of Thermo Field Dynamics in the Coulomb gauge
in order to investigate finite temperature effects on the atomic energy levels.
In the same context, we also analyze the problem of the ground state stability.Comment: LaTex file, 13 pages, no figures, includes PACS numbers and minor
changes in the text where a new section has been adde
Building a Sample of Distant Clusters of Galaxies
Candidate clusters of galaxies drawn from the sample identified from the
moderately deep I-band data of the ESO Imaging Survey (EIS), have been used for
follow-up optical/infrared imaging and spectroscopic observations. The
observations were conducted to assess the nature of these candidates over a
large range of redshifts. Currently, 163 EIS candidates have (V-I) colors, 15
have (I-K) and 65 cluster fields have been observed spectroscopically. From a
preliminary analysis of these data, we find that > 65% of the candidates
studied show strong evidence of being real physical associations, over the
redshift range 0.2<z<1.1. The evidence in some cases comes directly from
spectroscopic measurements, in others indirectly from the detection of
overdensities of objects with either the same color or the same photometric
redshift, or from a combination of color and spectroscopic information.
Preliminary results also suggest that the redshift derived from the
matched-filter algorithm is a reasonable measure of the cluster's redshift,
possibly overestimating it by Delta z ~0.1, at least for systems at z<0.7.
Overdensities of red objects have been detected in over 100 candidates, 38 of
which with estimated redshifts >0.6, and six candidates in the interval
0.45<z<0.81 have either been identified directly from measured redshifts or
have been confirmed by the measurement of at least one redshift for galaxies
located along a red-sequence typical of cluster early-type galaxies. Lastly,
five candidates among those already observed in the infrared have (I-Ks) colors
consistent with them being in the redshift interval 0.8<z<1.1. The sample of
"confirmed" clusters, already the largest of its kind in the southern
hemisphere, will be further enlarged by ongoing observations.Comment: To appear in "Large Scale Structure in the X-ray Universe", ed. M.
Plionis and I. Georgantopoulos (Paris: Editions Frontieres), in pres
in a hot and dense medium
The behavior of and in hot strange quark matter in weak
equilibrium with temperature, is investigated within the SU(3)
Nambu-Jona-Lasinio [NJL] model. Possible manifestations of restoration of
symmetries, by temperature or density, in the behavior of and
are discussed. The role played by the combined effect of temperature and
density in the nature of the phase transition and meson behavior is also
analyzed.Comment: 15 pages, 5 figures. Contribution to the Second International
Workshop on Hadron Physics, Effective Theories of Low Energy QCD, 25-29
September, 2002 (Coimbra, Portugal
Growth-Driven Percolations: The Dynamics of Community Formation in Neuronal Systems
The quintessential property of neuronal systems is their intensive patterns
of selective synaptic connections. The current work describes a physics-based
approach to neuronal shape modeling and synthesis and its consideration for the
simulation of neuronal development and the formation of neuronal communities.
Starting from images of real neurons, geometrical measurements are obtained and
used to construct probabilistic models which can be subsequently sampled in
order to produce morphologically realistic neuronal cells. Such cells are
progressively grown while monitoring their connections along time, which are
analysed in terms of percolation concepts. However, unlike traditional
percolation, the critical point is verified along the growth stages, not the
density of cells, which remains constant throughout the neuronal growth
dynamics. It is shown, through simulations, that growing beta cells tend to
reach percolation sooner than the alpha counterparts with the same diameter.
Also, the percolation becomes more abrupt for higher densities of cells, being
markedly sharper for the beta cells.Comment: 8 pages, 10 figure
Mass, angular-momentum, and charge inequalities for axisymmetric initial data
We present the key elements of the proof of an upper bound for
angular-momentum and charge in terms of the mass for electro-vacuum
asymptotically flat axisymmetric initial data sets with simply connected orbit
space
Phase transitions in quark matter and behaviour of physical observables in the vicinity of the critical end point
We study the chiral phase transition at finite and within the
framework of the SU(3) Nambu-Jona-Lasinio (NJL) model. The QCD critical end
point (CEP) and the critical line at finite temperature and baryonic chemical
potential are investigated: the study of physical quantities, such as the
baryon number susceptibility near the CEP, will provide complementary
information concerning the order of the phase transition. We also analyze the
information provided by the study of the critical exponents around the CEP.Comment: Talk given at the IVth International Conference on Quarks and Nuclear
Physics (QNP06), Madrid, 5-10 June 200
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