362 research outputs found
The gravitational energy-momentum flux
We present a continuity equation for the gravitational energy-momentum, which
is obtained in the framework of the teleparallel equivalent of general
relativity. From this equation it follows a general definition for the
gravitational energy-momentum flux. This definition is investigated in the
context of plane waves and of cylindrical Einstein-Rosen waves. We obtain the
well known value for the energy flux of plane gravitational waves, and conclude
that the latter exhibit features similar to plane electromagnetic waves.Comment: 20 pages, latex file, no figures, two references added, accepted for
publication in Class. Quantum Gravit
Dynamics and stability of the Godel universe
We use covariant techniques to describe the properties of the Godel universe
and then consider its linear response to a variety of perturbations. Against
matter aggregations, we find that the stability of the Godel model depends
primarily upon the presence of gradients in the centrifugal energy, and
secondarily on the equation of state of the fluid. The latter dictates the
behaviour of the model when dealing with homogeneous perturbations. The
vorticity of the perturbed Godel model is found to evolve as in almost-FRW
spacetimes, with some additional directional effects due to shape distortions.
We also consider gravitational-wave perturbations by investigating the
evolution of the magnetic Weyl component. This tensor obeys a simple plane-wave
equation, which argues for the neutral stability of the Godel model against
linear gravity-wave distortions. The implications of the background rotation
for scalar-field Godel cosmologies are also discussed.Comment: Revised version, to match paper published in Class. Quantum Gra
Hidden symmetries in the asymmetric exclusion process
We present a spectral study of the evolution matrix of the totally asymmetric
exclusion process on a ring at half filling. The natural symmetries
(translation, charge conjugation combined with reflection) predict only two
fold degeneracies. However, we have found that degeneracies of higher order
also exist and, as the system size increases, higher and higher orders appear.
These degeneracies become generic in the limit of very large systems. This
behaviour can be explained by the Bethe Ansatz and suggests the presence of
hidden symmetries in the model.
Keywords: ASEP, Markov matrix, symmetries, spectral degeneracies, Bethe
Ansatz.Comment: 16 page
Effect of atomic transfer on the decay of a Bose-Einstein condensate
We present a model describing the decay of a Bose-Einstein condensate, which
assumes the system to remain in thermal equilibrium during the decay. We show
that under this assumption transfer of atoms occurs from the condensate to the
thermal cloud enhancing the condensate decay rate
The Simon and Simon-Mars Tensors for Stationary Einstein-Maxwell Fields
Modulo conventional scale factors, the Simon and Simon-Mars tensors are
defined for stationary vacuum spacetimes so that their equality follows from
the Bianchi identities of the second kind. In the nonvacuum case one can absorb
additional source terms into a redefinition of the Simon tensor so that this
equality is maintained. Among the electrovacuum class of solutions of the
Einstein-Maxwell equations, the expression for the Simon tensor in the
Kerr-Newman-Taub-NUT spacetime in terms of the Ernst potential is formally the
same as in the vacuum case (modulo a scale factor), and its vanishing
guarantees the simultaneous alignment of the principal null directions of the
Weyl tensor, the Papapetrou field associated with the timelike Killing vector
field, the electromagnetic field of the spacetime and even the Killing-Yano
tensor.Comment: 12 pages, Latex IOP article class, no figure
Quantum optical time-of-arrival model in three dimensions
We investigate the three-dimensional formulation of a recently proposed
operational arrival-time model. It is shown that within typical conditions for
optical transitions the results of the simple one-dimensional version are
generally valid. Differences that may occur are consequences of Doppler and
momentum-transfer effects. Ways to minimize these are discussed.Comment: 14 pages, 5 figure
Electron transport through strongly interacting quantum dot coupled to normal metal and superconductor
We study the electron transport through the quantum dot coupled to the normal
metal and BCS-like superconductor (N - QD - S) in the presence of the Kondo
effect and Andreev scattering. The system is described by the single impurity
Anderson model in the limit of strong on-dot interaction. We use recently
proposed equation of motion technique for Keldysh nonequilibrium Green's
function together with the modified slave boson approach to study the electron
transport. We derive formula for the current which contains various tunneling
processes and apply it to study the transport through the system. We find that
the Andreev conductance is strongly suppressed and there is no zero-bias
(Kondo) anomaly in the differential conductance. We discuss effects of the
particle-hole asymmetry in the electrodes as well as the asymmetry in the
couplings.Comment: Supercond. Sci. Technol. - accepted for publicatio
Pseudorapidity Distributions of Charged Particles in d + Au and p + p Collisions at = 200GeV
The measured pseudorapidity distributions of primary charged particles are
presented for d + Au and p + p collisions at 200 GeV
over a wide pseudorapidity range of 5.4. The results
for d + Au collisions are presented for minimum-bias events and as a function
of collision centrality. The measurements for p + p collisions are shown for
minimum-bias events. The ratio of the charged particle multiplicity in d + Au
and p + A collisions relative to that for inelastic p + p collisions is found
to depend only on , and it is remarkably independent of
collision energy and system mass. The deuteron and gold fragmentation regions
in d + Au collisions are in good agreement with proton nucleus data at lower
energies.Comment: 4 pages, 3 figures. To appear in the proceedings of Seventeenth
International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions
(Quark Matter 2004), Oakland, California from January 11-17, 2004. Submitted
to Journal of Physics G: Nuclear and Particle Physic
Strangeness from 20 AGeV to 158 AGeV
New results from the energy scan programme of NA49, in particular kaon
production at 30 AGeV and phi production at 40 and 80 AGeV are presented. The
K+/pi+ ratio shows a pronounced maximum at 30 AGeV; the kaon slope parameters
are constant at SPS energies. Both findings support the scenario of a phase
transition at about 30 AGeV beam energy. The phi/pi ratio increases smoothly
with beam energy, showing an energy dependence similar to K-/pi-. The measured
particle yields can be reproduced by a hadron gas model, with chemical
freeze-out parameters on a smooth curve in the T-muB plane. The transverse
spectra can be understood as resulting from a rapidly expanding, locally
equilibrated source. No evidence for an earlier kinetic decoupling of heavy
hyperons is found.Comment: Contribution to the proceedings of "Strangeness in Quark Matter 2003"
(March 2003, Atlantic Beach NC, USA), to be published in Journal of Physics
G. 11 pages, 14 figure
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