201 research outputs found
Decoupling Limits in M-Theory
Limits of a system of N Dn-branes in which the bulk and string degrees of
freedom decouple to leave a `matter' theory are investigated and, for n>4,
either give a free theory or require taking . The decoupled
matter theory is described at low energies by the limit of n+1
dimensional \sym, and at high energies by a free type II string theory in a
curved space-time. Metastable bound states of D6-branes with mass and
D0-branes with mass are shown to have an energy proportional to
and decouple, whereas in matrix theory they only decouple in
the large N limit.Comment: 23 Pages, Tex, Phyzzx Macro. Minor correction
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
Crossover effects in a discrete deposition model with Kardar-Parisi-Zhang scaling
We simulated a growth model in 1+1 dimensions in which particles are
aggregated according to the rules of ballistic deposition with probability p or
according to the rules of random deposition with surface relaxation (Family
model) with probability 1-p. For any p>0, this system is in the
Kardar-Parisi-Zhang (KPZ) universality class, but it presents a slow crossover
from the Edwards-Wilkinson class (EW) for small p. From the scaling of the
growth velocity, the parameter p is connected to the coefficient of the
nonlinear term of the KPZ equation, lambda, giving lambda ~ p^gamma, with gamma
= 2.1 +- 0.2. Our numerical results confirm the interface width scaling in the
growth regime as W ~ lambda^beta t^beta, and the scaling of the saturation time
as tau ~ lambda^(-1) L^z, with the expected exponents beta =1/3 and z=3/2 and
strong corrections to scaling for small lambda. This picture is consistent with
a crossover time from EW to KPZ growth in the form t_c ~ lambda^(-4) ~ p^(-8),
in agreement with scaling theories and renormalization group analysis. Some
consequences of the slow crossover in this problem are discussed and may help
investigations of more complex models.Comment: 16 pages, 7 figures; to appear in Phys. Rev.
String-String Duality in Ten Dimensions
The heterotic string occurs as a soliton of the type I superstring in ten
dimensions, supporting the conjecture that these two theories are equivalent.
The conjecture that the type IIB string is self-dual, with the strong
coupling dynamics described by a dual type IIB theory, is supported by the
occurrence of the dual string as a Ramond-Ramond soliton of the weakly-coupled
theory.Comment: 14 pages, phyzz
A Space-Time Orbifold: A Toy Model for a Cosmological Singularity
We explore bosonic strings and Type II superstrings in the simplest time
dependent backgrounds, namely orbifolds of Minkowski space by time reversal and
some spatial reflections. We show that there are no negative norm physical
excitations. However, the contributions of negative norm virtual states to
quantum loops do not cancel, showing that a ghost-free gauge cannot be chosen.
The spectrum includes a twisted sector, with strings confined to a ``conical''
singularity which is localized in time. Since these localized strings are not
visible to asymptotic observers, interesting issues arise regarding unitarity
of the S-matrix for scattering of propagating states. The partition function of
our model is modular invariant, and for the superstring, the zero momentum
dilaton tadpole vanishes. Many of the issues we study will be generic to
time-dependent cosmological backgrounds with singularities localized in time,
and we derive some general lessons about quantizing strings on such spaces.Comment: 21 pages, 2 figure
Brane-anti-brane Democracy
We suggest a duality invariant formula for the entropy and temperature of
non-extreme black holes in supersymmetric string theory. The entropy is given
in terms of the duality invariant parameter of the deviation from extremality
and 56 SU(8) covariant central charges. It interpolates between the entropies
of Schwarzschild solution and extremal solutions with various amount of
unbroken supersymmetries and therefore serves for classification of black holes
in supersymmetric string theories. We introduce the second auxiliary 56 via
E(7) symmetric constraint. The symmetric and antisymmetric combinations of
these two multiplets are related via moduli to the corresponding two
fundamental representations of E(7): brane and anti-brane "numbers." Using the
CPT as well as C symmetry of the entropy formula and duality one can explain
the mysterious simplicity of the non-extreme black hole area formula in terms
of branes and anti-branes.Comment: LaTeX, 12 pages, no figure
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
Glucocorticoids prime the inflammatory response of human hippocampal cells through up-regulation of inflammatory pathways
Increased pro-inflammatory cytokines and an overactive hypothalamic-pituitary-adrenal (HPA) axis have both been implicated in the pathogenesis of depression. However, these explanations appear contradictory because glucocorticoids are well recognised for their anti-inflammatory effects. Two hypotheses exist to resolve this paradox: the mediating presence of glucocorticoid receptor resistance, or the possibility that glucocorticoids can potentiate inflammatory processes in some circumstances. We sought to investigate these hypotheses in a cell model with significant relevance to depression: human hippocampal progenitor cells. We demonstrated that dexamethasone in vitro given for 24 hours and followed by a 24 hours rest interval before an immune challenge potentiates inflammatory effects in these neural cells, that is, increases the IL-6 protein secretion induced by stimulation with IL-1\u3b2 (10 ng/mL for 24 hours) by + 49% (P < 0.05) at a concentration of 100 nM and by + 70% (P < 0.01) for 1 \u3bcM. These effects are time- and dose-dependent and require activation of the glucocorticoid receptor. Gene expression microarray assays using Human Gene 2.1st Array Strips demonstrated that glucocorticoid treatment up-regulated several innate immune genes, including chemokines and Nod-like receptor, NLRP6; using transcription factor binding motifs we found limited evidence that glucocorticoid resistance was induced in the cells. Our data suggests a mechanism by which stress may prime the immune system for increased inflammation and suggests that stress and inflammation may be synergistic in the pathogenesis of depression
Solitonic Strings and BPS Saturated Dyonic Black Holes
We consider a six-dimensional solitonic string solution described by a
conformal chiral null model with non-trivial superconformal transverse
part. It can be interpreted as a five-dimensional dyonic solitonic string wound
around a compact fifth dimension. The conformal model is regular with the
short-distance (`throat') region equivalent to a WZW theory. At distances
larger than the compactification scale the solitonic string reduces to a dyonic
static spherically-symmetric black hole of toroidally compactified heterotic
string. The new four-dimensional solution is parameterised by five charges,
saturates the Bogomol'nyi bound and has nontrivial dilaton-axion field and
moduli fields of two-torus. When acted by combined T- and S-duality
transformations it serves as a generating solution for all the static
spherically-symmetric BPS-saturated configurations of the low-energy heterotic
string theory compactified on six-torus. Solutions with regular horizons have
the global space-time structure of extreme Reissner-Nordstrom black holes with
the non-zero thermodynamic entropy which depends only on conserved (quantised)
charge vectors. The independence of the thermodynamic entropy on moduli and
axion-dilaton couplings strongly suggests that it should have a microscopic
interpretation as counting degeneracy of underlying string configurations. This
interpretation is supported by arguments based on the corresponding
six-dimensional conformal field theory. The expression for the level of the WZW
theory describing the throat region implies a renormalisation of the string
tension by a product of magnetic charges, thus relating the entropy and the
number of oscillations of the solitonic string in compact directions.Comment: 27 Pages, uses RevTeX (solution for the axion field corrected,
erratum to appear in Phys. Rev. D
In-medium relativistic kinetic theory and nucleon-meson systems
Within the model of coupled nucleon-meson systems, a
generalized relativistic Lenard--Balescu--equation is presented resulting from
a relativistic random phase approximation (RRPA). This provides a systematic
derivation of relativistic transport equations in the frame of nonequilibrium
Green's function technique including medium effects as well as flucuation
effects. It contains all possible processes due to one meson exchange and
special attention is kept to the off--shell character of the particles. As a
new feature of many particle effects, processes are possible which can be
interpreted as particle creation and annihilation due to in-medium one meson
exchange. In-medium cross sections are obtained from the generalized derivation
of collision integrals, which possess complete crossing symmetries.Comment: See nucl-th/9310032 for revised version which the authors
incompetently resubmitted rather than correctly replacing thi
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