471 research outputs found
Transport Properties of Quark and Gluon Plasmas
The kinetic properties of relativistic quark-gluon and electron-photon
plasmas are described in the weak coupling limit. The troublesome Rutherford
divergence at small scattering angles is screened by Debye screening for the
longitudinal or electric part of the interactions. The transverse or magnetic
part of the interactions is effectively screened by Landau damping of the
virtual photons and gluons transferred in the QED and QCD interactions
respectively. Including screening a number of transport coefficients for QCD
and QED plasmas can be calculated to leading order in the interaction strength,
including rates of momentum and thermal relaxation, electrical conductivity,
viscosities, flavor and spin diffusion of both high temperature and degenerate
plasmas. Damping of quarks and gluons as well as color diffusion in quark-gluon
plasmas is, however, shown not to be sufficiently screened and the rates
depends on an infrared cut-off of order the ``magnetic mass", .Comment: 12 pages, report LBL-3491
Freeze-out from HBT and Coulomb Effects
The freeze-out of hot and dense hadronic matter formed in relativistic
nuclear collisions is probed by HBT interferometry of identical pions, kaons,
etc. Coulomb repulsion/attraction of positive/negative particles show up at
small particle momenta and is also very sensitive to the freeze-out conditions.
The source sizes and times freeze-out are extracted from spectra
and HBT radii and compared.Comment: 4 pages, proc. of QM'97, Tsukuba, Japa
Hubbard model calculations of phase separation in optical lattices
Antiferromagnetic, Mott insulator, d-wave and gossamer superfluid phases are
calculated for 2D square lattices from the extended Hubbard (t-J-U) model using
the Gutzwiller projection method and renormalized mean field theory. Phase
separation between antiferromagnetic and d-wave superfluid phases is found near
half filling when the on-site repulsion exceeds U\ga7.3t, and coincides with
a first order transition in the double occupancy. Phase separation is thus
predicted for 2D optical lattices with ultracold Fermi atoms whereas it is
inhibited in cuprates by Coulomb frustration which instead may lead to stripes.
In a confined optical lattice the resulting density distribution is
discontinuous an with extended Mott plateau which enhances the
antiferromagnetic phase but suppresses the superfluid phase. Observation of
Mott insulator, antiferromagnetic, stripe and superfluid phases in density and
momentum distributions and correlations is discussed
Itinerant Ferromagnetism in ultracold Fermi gases
Itinerant ferromagnetism in cold Fermi gases with repulsive interactions is
studied applying the Jastrow-Slater approximation generalized to finite
polarization and temperature. For two components at zero temperature a second
order transition is found at compatible with QMC.
Thermodynamic functions and observables such as the compressibility and spin
susceptibility and the resulting fluctuations in number and spin are
calculated. For trapped gases the resulting cloud radii and kinetic energies
are calculated and compared to recent experiments. Spin polarized systems are
recommended for effective separation of large ferromagnetic domains. Collective
modes are predicted and tri-critical points are calculated for multi-component
systems.Comment: 8 pages, 6 figure
Bose-Einstein Correlations from Opaque Sources
Bose-Einstein correlations in relativistic heavy ion collisions are very
different for opaque sources than fortransparent ones. The Bose-Einstein radius
parameters measured in two-particle correlation functions depend sensitively on
the mean free path of the particles. In particular we find that the outward
radius for an opaque source is smaller than the sidewards radius for
sufficiently short duration of emission. A long duration of emission can
compensate the opacity reduction of the longitudinal radius parameter and
explain the experimental measurements of very similar side- and outward radius
parameters.Comment: additional material included, 8 pages, revtex, epsfig, 2 figure
included, manuscript also available at
http://www.nbi.dk/~vischer/publications.htm
Color, Spin and Flavor Diffusion in Quark-Gluon Plasmas
In weakly interacting quark-gluon plasmas diffusion of color is found to be
much slower than the diffusion of spin and flavor because color is easily
exchanged by the gluons in the very singular forward scattering processes. If
the infrared divergence is cut off by a magnetic mass, ,
the color diffusion is , a
factor smaller than spin and flavor diffusion. A similar effect is
expected in electroweak plasmas above due to exchanges. The color
conductivity in quark-gluon plasmas and the electrical conductivity in
electroweak plasmas are correspondingly small in relativistic heavy ion
collisions and the very early universe.Comment: 5 pages, no figure
Fluctuations and HBT Scales in Relativistic Nuclear Collisions
Bose-Einstein correlations in relativistic heavy ion collisions are examined
in a general model containing the essential features of hydrodynamical, cascade
as well as other models commonly employed for describing the particle
freeze-out. In particular the effects of longitudinal and transverse expansion,
emission from surfaces moving in time, the thickness of the emitting layer
varying from surface to volume emission and other effects are studied. Model
dependences of freeze-out sizes and times are discussed and compared to recent
data at 160AGeV.Comment: 9 pages, revtex, epsfig, 2 figure included, manuscript also available
at http://www.nbi.dk/~vischer/publications.htm
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