15,634 research outputs found
String Effects on Fermi--Dirac Correlation Measurements
We investigate some recent measurements of Fermi--Dirac correlations by the
LEP collaborations indicating surprisingly small source radii for the
production of baryons in -annihilation at the peak. In the
hadronization models there are besides the Fermi--Dirac correlation effect also
a strong dynamical (anti-)correlation. We demonstrate that the extraction of
the pure FD effect is highly dependent on a realistic Monte Carlo event
generator, both for separation of those dynamical correlations which are not
related to Fermi--Dirac statistics, and for corrections of the data and
background subtractions. Although the model can be tuned to well reproduce
single particle distributions, there are large model-uncertainties when it
comes to correlations between identical baryons. We therefore, unfortunately,
have to conclude that it is at present not possible to make any firm conclusion
about the source radii relevant for baryon production at LEP
Adiabatic radio frequency potentials for the coherent manipulation of matter waves
Adiabatic dressed state potentials are created when magnetic sub-states of
trapped atoms are coupled by a radio frequency field. We discuss their
theoretical foundations and point out fundamental advantages over potentials
purely based on static fields. The enhanced flexibility enables one to
implement numerous novel configurations, including double wells, Mach-Zehnder
and Sagnac interferometers which even allows for internal state-dependent atom
manipulation. These can be realized using simple and highly integrated wire
geometries on atom chips.Comment: 13 pages, 2 figure
r-modes in Relativistic Superfluid Stars
We discuss the modal properties of the -modes of relativistic superfluid
neutron stars, taking account of the entrainment effects between superfluids.
In this paper, the neutron stars are assumed to be filled with neutron and
proton superfluids and the strength of the entrainment effects between the
superfluids are represented by a single parameter . We find that the
basic properties of the -modes in a relativistic superfluid star are very
similar to those found for a Newtonian superfluid star. The -modes of a
relativistic superfluid star are split into two families, ordinary fluid-like
-modes (-mode) and superfluid-like -modes (-mode). The two
superfluids counter-move for the -modes, while they co-move for the
-modes. For the -modes, the quantity is
almost independent of the entrainment parameter , where and
are the azimuthal wave number and the oscillation frequency observed by an
inertial observer at spatial infinity, respectively. For the -modes, on
the other hand, almost linearly increases with increasing . It
is also found that the radiation driven instability due to the -modes is
much weaker than that of the -modes because the matter current associated
with the axial parity perturbations almost completely vanishes.Comment: 14 pages, 4 figures. To appear in Physical Review
Future geodesic completeness of some spatially homogeneous solutions of the vacuum Einstein equations in higher dimensions
It is known that all spatially homogeneous solutions of the vacuum Einstein
equations in four dimensions which exist for an infinite proper time towards
the future are future geodesically complete. This paper investigates whether
the analogous statement holds in higher dimensions. A positive answer to this
question is obtained for a large class of models which can be studied with the
help of Kaluza-Klein reduction to solutions of the Einstein-scalar field
equations in four dimensions. The proof of this result makes use of a criterion
for geodesic completeness which is applicable to more general spatially
homogeneous models.Comment: 18 page
Stellar Pulsations excited by a scattered mass
We compute the energy spectra of the gravitational signals emitted when a
mass m is scattered by the gravitational field of a star of mass M >> m. We
show that, unlike black holes in similar processes, the quasi-normal modes of
the star are excited, and that the amount of energy emitted in these modes
depends on how close the exciting mass can get to the star.Comment: 23 pages, 6 figures, RevTe
R-mode Instability of Slowly Rotating Non-isentropic Relativistic Stars
We investigate properties of -mode instability in slowly rotating
relativistic polytropes. Inside the star slow rotation and low frequency
formalism that was mainly developed by Kojima is employed to study axial
oscillations restored by Coriolis force. At the stellar surface, in order to
take account of gravitational radiation reaction effect, we use a near-zone
boundary condition instead of the usually imposed boundary condition for
asymptotically flat spacetime. Due to the boundary condition, complex
frequencies whose imaginary part represents secular instability are obtained
for discrete -mode oscillations in some polytropic models. It is found that
such discrete -mode solutions can be obtained only for some restricted
polytropic models. Basic properties of the solutions are similar to those
obtained by imposing the boundary condition for asymptotically flat spacetime.
Our results suggest that existence of a continuous part of spectrum cannot be
avoided even when its frequency becomes complex due to the emission of
gravitational radiation.Comment: 10 pages, 4 figures, accepted for publlication in PR
The rotational modes of relativistic stars: Numerical results
We study the inertial modes of slowly rotating, fully relativistic compact
stars. The equations that govern perturbations of both barotropic and
non-barotropic models are discussed, but we present numerical results only for
the barotropic case. For barotropic stars all inertial modes are a hybrid
mixture of axial and polar perturbations. We use a spectral method to solve for
such modes of various polytropic models. Our main attention is on modes that
can be driven unstable by the emission of gravitational waves. Hence, we
calculate the gravitational-wave growth timescale for these unstable modes and
compare the results to previous estimates obtained in Newtonian gravity (i.e.
using post-Newtonian radiation formulas). We find that the inertial modes are
slightly stabilized by relativistic effects, but that previous conclusions
concerning eg. the unstable r-modes remain essentially unaltered when the
problem is studied in full general relativity.Comment: RevTeX, 29 pages, 31 eps figure
Stability for linearized gravity on the Kerr spacetime
In this paper we prove integrated energy and pointwise decay estimates for solutions of the vacuum linearized Einstein equation on the Kerr black hole exterior. The estimates are valid for the full, subextreme range of Kerr black holes, provided integrated energy estimates for the Teukolsky Master Equation holds. For slowly rotating Kerr backgrounds, such estimates are known to hold, due to the work of one of the authors arXiv:1708.07385. The results in this paper thus provide the first stability results for linearized gravity on the Kerr background, in the slowly rotating case, and reduce the linearized stability problem for the full subextreme range to proving integrated energy estimates for the Teukolsky equation. This constitutes an essential step towards a proof of the black hole stability conjecture, i.e. the statement that the Kerr family is dynamically stable, one of the central open problems in general relativity. The proof relies on three key steps. First, there are energy decay estimates for the Teukolsky equation, proved by applying weighted multiplier estimates to a system of spin-weighted wave equations derived from the Teukolsky equation, and making use of the pigeonhole principle for the resulting hierarchy of weighted energy estimates. Second, working in the outgoing radiation gauge, the linearized Einstein equations are written as a system of transport equations, driven by one of the Teukolsky scalars. Third, expansions for the relevant curvature, connection, and metric components can be made near null infinity. An analysis of the dynamics on future null infinity, together with the Teukolsky Starobinsky Identity plays an important role in the argument
Disorder Potentials near Lithographically Fabricated Atom Chips
We show that previously observed large disorder potentials in magnetic
microtraps for neutral atoms are reduced by about two orders of magnitude when
using atom chips with lithographically fabricated high quality gold layers.
Using one dimensional Bose-Einstein condensates, we probe the remaining
magnetic field variations at surface distances down to a few microns.
Measurements on a 100 um wide wire imply that residual variations of the
current flow result from local properties of the wire.Comment: submitted on September 24th, 200
On marginally outer trapped surfaces in stationary and static spacetimes
In this paper we prove that for any spacelike hypersurface containing an
untrapped barrier in a stationary spacetime satisfying the null energy
condition, any marginally outer trapped surface cannot lie in the exterior
region where the stationary Killing vector is timelike. In the static case we
prove that any marginally outer trapped surface cannot penetrate into the
exterior region where the static Killing vector is timelike. In fact, we prove
these result at an initial data level, without even assuming existence of a
spacetime. The proof relies on a powerful theorem by Andersson and Metzger on
existence of an outermost marginally outer trapped surface.Comment: 22 pages, 3 figures; 1 reference added, 1 figure changed, other minor
change
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