8,761 research outputs found
A Transport Equation for Quantum Fields with Continuous Mass Spectrum
Within a relativistic real-time Green's function formalism, a quantum
transport equation for the phase-space distribution function is derived without
a quasi-particle approximation. Dissipation is due to a nonzero spectral width,
and can be separated into time-local and memory effects.Comment: 25 pages LaTeX, 1 figure uuencoded, GSI-Preprint 94-1
Quantum Fields and Dissipation
The description of thermal or non-equilibrium systems necessitates a quantum
field theory which differs from the usual approach in two aspects: 1.The
Hilbert space is doubled; 2.Stable quasi-particles do not exist in interacting
systems. A mini-review of these two aspects is given from a practical viewpoint
including two applications. For thermal states it is shown how infrared
divergences occuring in perturbative quasi-particle theories are avoided,
whereas for non-equilibrium states a memory effect is shown to arise in the
thermalization.Comment: Paper in ReVTeX, 12 pages. Figures and complete paper available via
anonymous ftp at ftp://tpri6c.gsi.de/pub/phenning/h96neq, contribution to the
Umezawa memorial volume of Physics Essay
Clustered Star Formation in the Small Magellanic Cloud. A Spitzer/IRAC View of the Star-Forming Region NGC 602/N 90
We present Spitzer/IRAC photometry on the star-forming HII region N 90,
related to the young stellar association NGC 602 in the Small Magellanic Cloud.
Our photometry revealed bright mid-infrared sources, which we classify with the
use of a scheme based on templates and models of red sources in the Milky Way,
and criteria recently developed from the Spitzer Survey of the SMC for the
selection of candidate Young Stellar Objects (YSOs). We detected 57 sources in
all four IRAC channels in a 6.2' x 4.8' field-of-view centered on N 90; 22 of
these sources are classified as candidate YSOs. We compare the locations of
these objects with the position of optical sources recently found in the same
region with high-resolution HST/ACS imaging of NGC 602, and we find that 17
candidate YSOs have one or more optical counterparts. All of these optical
sources are identified as pre-main sequence stars, indicating, thus, ongoing
clustered star formation events in the region. The positions of the detected
YSOs and their related PMS clusters give a clear picture of the current star
formation in N 90, according to which the young stellar association
photo-ionizes the surrounding interstellar medium, revealing the HII nebula,
and triggering sequential star formation events mainly along the eastern and
southern rims of the formed cavity of the parental molecular cloud.Comment: Accepted fro Publication in ApJ. 8 pages, 6 figures, 3 color figures
submitted as JP
Meson and Quark Degrees of Freedom and the Radius of the Deuteron
The existing experimental data for the deuteron charge radius are discussed.
The data of elastic electron scattering are inconsistent with the value
obtained in a recent atomic physics experiment. Theoretical predictions based
on a nonrelativistic description of the deuteron with realistic nucleon-nucleon
potentials and with a rather complete set of meson-exchange contributions to
the charge operator are presented. Corrections arising from the quark-gluon
substructure of the nucleon are explored in a nonrelativistic quark model; the
quark-gluon corrections, not accounted for by meson exchange, are small. Our
prediction for the deuteron charge radius favors the value of a recent atomic
physics experiment.Comment: 20 pages, LaTeX, 4 Postscript figures, to appear in Few-Body-System
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