180 research outputs found
On symmetries of Chern-Simons and BF topological theories
We describe constructing solutions of the field equations of Chern-Simons and
topological BF theories in terms of deformation theory of locally constant
(flat) bundles. Maps of flat connections into one another (dressing
transformations) are considered. A method of calculating (nonlocal) dressing
symmetries in Chern-Simons and topological BF theories is formulated
String Form Factors
We compute the cross section for scattering of light string probes by
randomly excited closed strings. For high energy probes, the cross section
factorizes and can be used to define effective form factors for the excited
targets. These form factors are well defined without the need for infinite
subtractions and contain information about the shape and size of typical
strings. For highly excited strings the elastic form factor can be written in
terms of the `plasma dispersion function', which describes charge screening in
high temperature plasmas.Comment: 18 pages, 3 figures. Typos corrected, 1 footnote (in Section 4) and 1
reference adde
Entanglement and entropy operator for strings in pp-wave time dependent background
In this letter new aspects of string theory propagating in a pp-wave time
dependent background with a null singularity are explored. It is shown the
appearance of a 2d entanglement entropy dynamically generated by the
background. For asymptotically flat observers, the vacuum close to the
singularity is unitarily inequivalent to the vacuum at and it
is shown that the 2d entanglement entropy diverges close to this point. As a
consequence, the positive time region is inaccessible for observers in . For a stationary measure, the vacuum at finite time is seen by those
observers as a thermal state and the information loss is encoded as a heat bath
of string states.Comment: revtex4, 15 pages, revised version to appear in Physics Letters
Self-consistent description of nuclear compressional modes
Isoscalar monopole and dipole compressional modes are computed for a variety
of closed-shell nuclei in a relativistic random-phase approximation to three
different parametrizations of the Walecka model with scalar self-interactions.
Particular emphasis is placed on the role of self-consistency which by itself,
and with little else, guarantees the decoupling of the spurious
isoscalar-dipole strength from the physical response and the conservation of
the vector current. A powerful new relation is introduced to quantify the
violation of the vector current in terms of various ground-state form-factors.
For the isoscalar-dipole mode two distinct regions are clearly identified: (i)
a high-energy component that is sensitive to the size of the nucleus and scales
with the compressibility of the model and (ii) a low-energy component that is
insensitivity to the nuclear compressibility. A fairly good description of both
compressional modes is obtained by using a ``soft'' parametrization having a
compression modulus of K=224 MeV.Comment: 28 pages and 10 figures; submitted to PR
Observer dependent D-brane for strings propagating in pp-wave time dependent background
We study type IIB superstring in the pp-wave time-dependent background, which
has a singularity at . We show that this background can provide a toy
model to study some ideas related to the stretched horizon paradigm and the
complementary principle of black holes. To this end, we construct a unitary
Bogoliubov generator which relates the asymptotically flat string Hilbert
space, defined at , to the finite time Hilbert space. For
asymptotically flat observers, the closed string vacuum close to the
singularity appears as a boundary state which is in fact a D-brane described in
the closed string channel. However, observers who go with the string towards to
the singularity see the original vacuum.Comment: 12 pages, revtex 4, added references, corrected mistake
Comparison of and Quasielastic Scattering
We formulate -nucleus quasielastic scattering in a manner which closely
parallels standard treatments of -nucleus quasielastic scattering. For
scattering, new responses involving scalar contributions appear in
addition to the Coulomb (or longitudinal) and transverse responses
which are of vector character. We compute these responses using both nuclear
matter and finite nucleus versions of the Relativistic Hartree Approximation to
Quantum Hadrodynamics including RPA correlations. Overall agreement with
measured responses and new quasielastic scattering data for
Ca at |\qs|=500 MeV/c is good. Strong RPA quenching is essential for
agreement with the Coulomb response. This quenching is notably less for the
cross section even though the new scalar contributions are even more
strongly quenched than the vector contributions. We show that this
``differential quenching'' alters sensitive cancellations in the expression for
the cross section so that it is reduced much less than the individual
responses. We emphasize the role of the purely relativistic distinction between
vector and scalar contributions in obtaining an accurate and consistent
description of the and data within the framework of our nuclear
structure model.Comment: 26 pages, 5 uuencoded figures appended to end of this fil
Density Dependent Hadron Field Theory
A fully covariant approach to a density dependent hadron field theory is
presented. The relation between in--medium NN interactions and
field--theoretical meson--nucleon vertices is discussed. The medium dependence
of nuclear interactions is described by a functional dependence of the
meson--nucleon vertices on the baryon field operators. As a consequence, the
Euler--Lagrange equations lead to baryon rearrangement self--energies which are
not obtained when only a parametric dependence of the vertices on the density
is assumed. It is shown that the approach is energy--momentum conserving and
thermodynamically consistent. Solutions of the field equations are studied in
the mean--field approximation. Descriptions of the medium dependence in terms
of the baryon scalar and vector density are investigated. Applications to
infinite nuclear matter and finite nuclei are discussed. Density dependent
coupling constants obtained from Dirac--Brueckner calculations with the Bonn
NN-potentials are used. Results from Hartree calculations for energy spectra,
binding energies and charge density distributions of , and
are presented. Comparisons to data strongly support the importance
of rearrangement in a relativistic density dependent field theory. Most
striking is the simultanuous improvement of charge radii, charge densities and
binding energies. The results indicate the appearance of a new "Coester line"
in the nuclear matter equation of state.Comment: 48 LateX pages, 12 Figures, figures and full paper are available as
postscript files by anonymous ftp at ftp://theorie.physik.uni-giessen.de/dd
Anatomy of nuclear shape transition in the relativistic mean field theory
A detailed microscopic study of the temperature dependence of the shapes of
some rare-earth nuclei is made in the relativistic mean field theory. Analyses
of the thermal evolution of the single-particle orbitals and their occupancies
leading to the collapse of the deformation are presented. The role of the
non-linear field on the shape transition in different nuclei is also
investigated; in its absence the shape transition is found to be sharper.Comment: REVTEX file (13pages), 12 figures, Phys. Rev. C(in press),
\documentstyle[aps,preprint]{revtex
Medium Modification to the -Meson Mass in the Walecka Model
We calculate the effective mass of the meson in nuclear matter in a
relativistic random-phase approximation to the Walecka model. The dressing of
the meson propagator is driven by its coupling to particle-hole pairs and
nucleon-antinucleon () excitations. We report a reduction in the
-meson mass of about 170~MeV at nuclear-matter saturation density. This
reduction arises from a competition between the density-dependent
(particle-hole) dressing of the propagator and vacuum polarization (
pairs). While density-dependent effects lead to an increase in the mass
proportional to the classical plasma frequency, vacuum polarization leads to an
even larger reduction caused by the reduced effective nucleon mass in the
medium.Comment: 14 pages in ReVTeX, 3 uuencoded figures are available upon request,
FSU-SCRI-93-132 and ADP-93-223/T14
Relativistic Coulomb Sum Rules for
A Coulomb sum rule is derived for the response of nuclei to
scattering with large three-momentum transfers. Unlike the nonrelativistic
formulation, the relativistic Coulomb sum is restricted to spacelike
four-momenta for the most direct connection with experiments; an immediate
consequence is that excitations involving antinucleons, e.g., pair
production, are approximately eliminated from the sum rule. Relativistic recoil
and Fermi motion of target nucleons are correctly incorporated. The sum rule
decomposes into one- and two-body parts, with correlation information in the
second. The one-body part requires information on the nucleon momentum
distribution function, which is incorporated by a moment expansion method. The
sum rule given through the second moment (RCSR-II) is tested in the Fermi gas
model, and is shown to be sufficiently accurate for applications to data.Comment: 32 pages (LaTeX), 4 postscript figures available from the author
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