1,143 research outputs found

### Exclusive electromagnetic production of strangeness on the nucleon : review of recent data in a Regge approach

In view of the numerous experimental results recently released, we provide in
this letter an update on the performance of our simple Regge model for
strangeness electroproduction on the nucleon. Without refitting any parameters,
a decent description of all measured observables and channels is achieved. We
also give predictions for spin transfer observables, recently measured at
Jefferson Lab which have high sensitivity to discriminate between different
theoretical approaches.Comment: 5 pages, 5 figure

### A Gravitational Effective Action on a Finite Triangulation

We construct a function of the edge-lengths of a triangulated surface whose
variation under a rescaling of all the edges that meet at a vertex is the
defect angle at that vertex. We interpret this function as a gravitational
effective action on the triangulation, and the variation as a trace anomaly.Comment: 5 pages; clarifications, acknowledgements, references adde

### Unified Brane Gravity: Cosmological Dark Matter from Scale Dependent Newton Constant

We analyze, within the framework of unified brane gravity, the weak-field
perturbations caused by the presence of matter on a 3-brane. Although deviating
from the Randall-Sundrum approach, the masslessness of the graviton is still
preserved. In particular, the four-dimensional Newton force law is recovered,
but serendipitously, the corresponding Newton constant is shown to be
necessarily lower than the one which governs FRW cosmology. This has the
potential to puzzle out cosmological dark matter. A subsequent conjecture
concerning galactic dark matter follows.Comment: 6 pages, to be published in Phys. Rev.

### Degeneracies when T=0 Two Body Interacting Matrix Elements are Set Equal to Zero : Talmi's method of calculating coefficients of fractional parentage to states forbidden by the Pauli principle

In a previous work we studied the effects of setting all two body T=0 matrix
elements to zero in shell model calculations for $^{43}$Ti ($^{43}$Sc) and
$^{44}$Ti. The results for $^{44}$Ti were surprisingly good despite the
severity of this approximation. In this approximation degeneracies arose in the
T=1/2 I=$({1/2})^-_1$ and $({13/2})^-_1$ states in $^{43}$Sc and the T=1/2
$I=({13/2})_2^-$, $({17/2})^-_1$, and $({19/2})_1^-$ in $^{43}$Sc. The T=0
$3_2^+$, $7_2^+$, $9_1^+$, and $10_1^+$ states in $^{44}$Ti were degenerate as
well. The degeneracies can be explained by certain 6j symbols and 9j symbols
either vanishing or being equal as indeed they are. Previously we used Regge
symmetries of 6j symbols to explain these degeneracies. In this work a simpler
more physical method is used. This is Talmi's method of calculating
coefficients of fractional parentage for identical particles to states which
are forbidden by the Pauli principle. This is done for both one particle cfp to
handle 6j symbols and two particle cfp to handle 9j symbols. The states can be
classified by the dual quantum numbers ($J_{\pi},J_{\nu}$)

### Gravity action on the rapidly varying metrics

We consider a four-dimensional simplicial complex and the minisuperspace
general relativity system described by the metric flat in the most part of the
interior of every 4-simplex with exception of a thin layer of thickness
$\propto \varepsilon$ along the every three-dimensional face where the metric
undergoes jump between the two 4-simplices sharing this face. At $\varepsilon
\to 0$ this jump would become discontinuity. Since, however, discontinuity of
the (induced on the face) metric is not allowed in general relativity, the
terms in the Einstein action tending to infinity at $\varepsilon \to 0$ arise.
In the path integral approach, these terms lead to the pre-exponent factor with
\dfuns requiring that the induced on the faces metric be continuous, i. e. the
4-simplices fit on their common faces. The other part of the path integral
measure corresponds to the action being the sum of independent terms over the
4-simplices. Therefore this part of the path integral measure is the product of
independent measures over the 4-simplices. The result obtained is in accordance
with our previous one obtained from the symmetry considerations.Comment: 10 page

### Finite size scaling in Villain's fully frustrated model and singular effects of plaquette disorder

The ground state and low T behavior of two-dimensional spin systems with
discrete binary couplings are subtle but can be analyzed using exact
computations of finite volume partition functions. We first apply this approach
to Villain's fully frustrated model, unveiling an unexpected finite size
scaling law. Then we show that the introduction of even a small amount of
disorder on the plaquettes dramatically changes the scaling laws associated
with the T=0 critical point.Comment: Latex with 3 ps figures. Last versio

### Invariants of 2+1 Quantum Gravity

In [1,2] we established and discussed the algebra of observables for 2+1
gravity at both the classical and quantum level. Here our treatment broadens
and extends previous results to any genus $g$ with a systematic discussion of
the centre of the algebra. The reduction of the number of independent
observables to $6g-6 (g > 1)$ is treated in detail with a precise
classification for $g = 1$ and $g = 2$.Comment: 10 pages, plain TEX, no figures, DFTT 46/9

### Multi-Dimensional Astrophysical Structural and Dynamical Analysis I. Development of a Nonlinear Finite Element Approach

A new field of numerical astrophysics is introduced which addresses the
solution of large, multidimensional structural or slowly-evolving problems
(rotating stars, interacting binaries, thick advective accretion disks, four
dimensional spacetimes, etc.). The technique employed is the Finite Element
Method (FEM), commonly used to solve engineering structural problems. The
approach developed herein has the following key features:
1. The computational mesh can extend into the time dimension, as well as
space, perhaps only a few cells, or throughout spacetime.
2. Virtually all equations describing the astrophysics of continuous media,
including the field equations, can be written in a compact form similar to that
routinely solved by most engineering finite element codes.
3. The transformations that occur naturally in the four-dimensional FEM
possess both coordinate and boost features, such that
(a) although the computational mesh may have a complex, non-analytic,
curvilinear structure, the physical equations still can be written in a simple
coordinate system independent of the mesh geometry.
(b) if the mesh has a complex flow velocity with respect to coordinate space,
the transformations will form the proper arbitrary Lagrangian- Eulerian
advective derivatives automatically.
4. The complex difference equations on the arbitrary curvilinear grid are
generated automatically from encoded differential equations.
This first paper concentrates on developing a robust and widely-applicable
set of techniques using the nonlinear FEM and presents some examples.Comment: 28 pages, 9 figures; added integral boundary conditions, allowing
very rapidly-rotating stars; accepted for publication in Ap.

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