79 research outputs found
Comments on spin operators and spin-polarization states of 2+1 fermions
In this brief article we discuss spin polarization operators and spin
polarization states of 2+1 massive Dirac fermions and find a convenient
representation by the help of 4-spinors for their description. We stress that
in particular the use of such a representation allows us to introduce the
conserved covariant spin operator in the 2+1 field theory. Another advantage of
this representation is related to the pseudoclassical limit of the theory.
Indeed, quantization of the pseudoclassical model of a spinning particle in 2+1
dimensions leads to the 4-spinor representation as the adequate realization of
the operator algebra, where the corresponding operator of a first-class
constraint, which cannot be gauged out by imposing the gauge condition, is just
the covariant operator previously introduced in the quantum theory.Comment: 6 page
Energy-momentum tensor in thermal strong-field QED with unstable vacuum
The mean value of the one-loop energy-momentum tensor in thermal QED with
electric-like background that creates particles from vacuum is calculated. The
problem differes essentially from calculations of effective actions (similar to
that of Heisenberg--Euler) in backgrounds that do not violate the stability of
vacuum. The role of a constant electric background in the violation of both the
stability of vacuum and the thermal character of particle distribution is
investigated. Restrictions on the electric field and its duration under which
one can neglect the back-reaction of created particles are established.Comment: 7 pages, Talk presented at Workshop "Quantum Field Theory under the
Influence of External Conditions", Leipzig, September 17-21, 2007;
introduction extended, version accepted for publication in J.Phys.
Vacuum Polarization Effects in the Lorentz and PCT Violating Electrodynamics
In this work we report new results concerning the question of dynamical mass
generation in the Lorentz and PCT violating quantum electrodynamics. A one loop
calculation for the vacuum polarization tensor is presented. The electron
propagator, "dressed" by a Lorentz breaking extra term in the fermion
Lagrangian density, is approximated by its first order: this scheme is shown to
break gauge invariance. Then we rather consider a full calculation to second
order in the Lorentz breaking parameter: we recover gauge invariance and use
the Schwinger-Dyson equation to discuss the full photon propagator. This allows
a discussion on a possible photon mass shift as well as measurable, observable
physical consequences, such as the Lamb-shift.Comment: Latex file, 19 pages, no figures, includes PACS number
Utilização de fontes de energia renovåveis para o aquecimento de aves.
Projeto/Plano de Ação: 03.05.01.021
EmisiĂłn acumulada de Ăłxido nitroso (N20) en sistemas pastoriles de la Sierra Catarinense - Brasil.
En este trabajo se evaluaron y compararon la emisiĂłn de N2O generados por los sistemas pastoriles mĂĄs empleados en Santa Catarina, Brasil.Suplemento 1. Edição do 43Âș Congreso Argentino de ProducciĂłn Animal
Incised valley paleoenvironments interpreted by seismic stratigraphic approach in Patos Lagoon, Southern Brazil
<div><p>ABSTRACT: The Rio Grande do Sul (RS) coastal plain area (33,000 km 2 ) had its physiography modified several times through the Quaternary, responding to allogenic and autogenic forcings. The Patos Lagoon covers a significant area of RS coastal plain (10,000 km 2 ), where incised valleys were identified in previous works. About 1,000 km of high resolution (3.5 kHz) seismic profiles, radiocarbon datings, Standard Penetration Test (SPT) and gravity cores were analyzed to interpret the paleoenvironmental evolution as preserved in incised valley infills. Seismic facies were recognized by seismic parameters. The sediment cores were used to ground-truth the seismic interpretations and help in the paleoenvironmental identification. Key surfaces were established to detail the stratigraphical framework, and seismic facies were grouped into four seismic units, which one classified in respective system tracts within three depositional sequences. The oldest preserved deposits are predominantly fluvial and estuarine facies, representing the falling stage and lowstand system tracts. The Holocene transgressive records are dominated by muddy material, mainly represented by estuarine facies with local variations. The transgression culminated in Late Holocene deposits of Patos Lagoon, representing the highstand system tract. The depositional pattern of the vertical succession was controlled by eustatic variations, while the autogenic forcing (paleogeography and sediment supply) modulated the local facies variation.</p></div
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