2,225 research outputs found
3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory
An effective field theory for matter coupled to three-dimensional quantum
gravity was recently derived in the context of spinfoam models in
hep-th/0512113. In this paper, we show how this relates to group field theories
and generalized matrix models. In the first part, we realize that the effective
field theory can be recasted as a matrix model where couplings between matrices
of different sizes can occur. In a second part, we provide a family of
classical solutions to the three-dimensional group field theory. By studying
perturbations around these solutions, we generate the dynamics of the effective
field theory. We identify a particular case which leads to the action of
hep-th/0512113 for a massive field living in a flat non-commutative space-time.
The most general solutions lead to field theories with non-linear redefinitions
of the momentum which we propose to interpret as living on curved space-times.
We conclude by discussing the possible extension to four-dimensional spinfoam
models.Comment: 17 pages, revtex4, 1 figur
Relaxing Cosmological Constraints on Large Extra Dimensions
We reconsider cosmological constraints on extra dimension theories from the
excess production of Kaluza-Klein gravitons. We point out that, if the normalcy
temperature is above 1 GeV, then graviton states produced at this temperature
will decay early enough that they do not affect the present day dark matter
density, or the diffuse gamma ray background. We rederive the relevant
cosmological constraints for this scenario.Comment: 17 pages, latex, revtex4; added a short discussion of other
constraints, reference
A proteomic analysis of the ventral and dorsal hippocampal brain areas of serotonin knockout rats
Thesis (MScMedSc (Biomedical Sciences. Medical Physiology)--Stellenbosch University, 2008.For many centuries, scientists have engaged in a theoretical debate concerning the etiology
of mood disorders, with very few ancient scholars speculating about the importance of
genetic factors and affective temperaments as factors in the etiology of depression. Mood,
emotion and cognition have been shown to be modulated by the serotonergic midbrain
raphe system; implicated in the pathogenesis of psychiatric disorders like those of the
affective spectrum. Evidence from neuroscience, genetics, and clinical investigation
demonstrate that depression is a disorder of the brain. Brain imaging research is revealing
that in depression, neural circuits responsible for moods, thinking, sleep, appetite, and
behavior fail to function properly, and that the regulation of critical neurotransmitters is
impaired. Genetics research, including studies of twins, indicates that genes play a role in
depression. Vulnerability to depression appears to result from the influence of multiple genes
acting together with environmental factors. Other research has shown that stressful life
events, particularly in the form of loss such as the death of a close family member, may
trigger major depression in susceptible individuals. Depression and anxiety have often been
successfully treated by means of selective serotonin reuptake inhibitors. However, selective
serotonin reuptake inhibitors do not solve all the problems inherent to the treatment of
depression, for approximately 30 % of depressed patients do not respond to treatment and
20 % experience relapses whilst on treatment. Of consideration is the fact that the majority
of drugs today are based on proteins, with 50 % of therapeutics on the market targeting cell
membrane proteins. Up to this day the precise pathophysiology of mood disorders remains
obscure, as does the neurobiology of normal mood regulation. Accordingly, there is a need
for methods to identify the structural and/or signaling components which lead to changes in
the brain, particularly the hippocampus, of subjects having mood disorders such as bipolar
depressive disorder, chronic major depressive disorder and the like. Similarly, there is a
need for the early detection, screening and diagnosis of individuals at risk for a mood
disorder. As the serotonin tranpsorter is the primary target for therapeutic intervention in the
treatment of numerous psychiatric disorders and considering the fact that at the structural
level this protein’s function as transporter in membranes remains incompletely understood,
investigating its function in psychiatric disorders are of importance . The objective of this
study was to determine the role of the serotonin transporter in wild type and serotonin
knockout rats, with regards to the hippocampus. Rat hippocampi were fractionated into
cytosolic and membrane components, which were run and further separated in two
dimensions. Firstly separation occurred by isoelectrical focusing (pI), follwed by gel
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electrophoresis (molecular weight). Gels were compared to see whether protein spots have
changed between animals that have been differentially bred. Differentially expressed protein
spots, as determined by PD Quest software, were excised, digested and analyzed by means
of mass spectrometry. Our results indicated that metabolic, structural and cell signaling
proteins were differentially expressed in both the ventral and dorsal hippocampus of the
serotonin knockout rat. Futhermore, cellular stress proteins were found to be only
differentially expressed in the ventral hippocampus. The majority of proteins identified in
both hippocampal areas as well as both fractions, were assigned to energy metabolism. The
cytosolic protein profile mirrored the pattern of the membrane protein profile. In conclusion,
this proteomic study identified various protein groups that interacted with one another, thus
establishing compensation for disrupted serotonin homeostasis
Observables in 3d spinfoam quantum gravity with fermions
We study expectation values of observables in three-dimensional spinfoam
quantum gravity coupled to Dirac fermions. We revisit the model introduced by
one of the authors and extend it to the case of massless fermionic fields. We
introduce observables, analyse their symmetries and the corresponding proper
gauge fixing. The Berezin integral over the fermionic fields is performed and
the fermionic observables are expanded in open paths and closed loops
associated to pure quantum gravity observables. We obtain the vertex amplitudes
for gauge-invariant observables, while the expectation values of gauge-variant
observables, such as the fermion propagator, are given by the evaluation of
particular spin networks.Comment: 32 pages, many diagrams, uses psfrag
Euclidean three-point function in loop and perturbative gravity
We compute the leading order of the three-point function in loop quantum
gravity, using the vertex expansion of the Euclidean version of the new spin
foam dynamics, in the region of gamma<1. We find results consistent with Regge
calculus in the limit gamma->0 and j->infinity. We also compute the tree-level
three-point function of perturbative quantum general relativity in position
space, and discuss the possibility of directly comparing the two results.Comment: 16 page
Effective action and semiclassical limit of spin foam models
We define an effective action for spin foam models of quantum gravity by
adapting the background field method from quantum field theory. We show that
the Regge action is the leading term in the semi-classical expansion of the
spin foam effective action if the vertex amplitude has the large-spin
asymptotics which is proportional to an exponential function of the vertex
Regge action. In the case of the known three-dimensional and four-dimensional
spin foam models this amounts to modifying the vertex amplitude such that the
exponential asymptotics is obtained. In particular, we show that the ELPR/FK
model vertex amplitude can be modified such that the new model is finite and
has the Einstein-Hilbert action as its classical limit. We also calculate the
first-order and some of the second-order quantum corrections in the
semi-classical expansion of the effective action.Comment: Improved presentation, 2 references added. 15 pages, no figure
Limit on the mass of a long-lived or stable gluino
We reinterpret the generic CDF charged massive particle limit to obtain a
limit on the mass of a stable or long-lived gluino. Various sources of
uncertainty are examined. The -hadron spectrum and scattering cross sections
are modeled based on known low-energy hadron physics and the resultant
uncertainties are quantified and found to be small compared to uncertainties
from the scale dependence of the NLO pQCD production cross sections. The
largest uncertainty in the limit comes from the unknown squark mass: when the
squark -- gluino mass splitting is small, we obtain a gluino mass limit of 407
GeV, while in the limit of heavy squarks the gluino mass limit is 397 GeV. For
arbitrary (degenerate) squark masses, we obtain a lower limit of 322 GeV on the
gluino mass. These limits apply for any gluino lifetime longer than
ns, and are the most stringent limits for such a long-lived or stable gluino.Comment: 15 pages, 5 figures, accepted for publication in JHE
The Zero Age Main Sequence of WIMP burners
We modify a stellar structure code to estimate the effect upon the main
sequence of the accretion of weakly interacting dark matter onto stars and its
subsequent annihilation. The effect upon the stars depends upon whether the
energy generation rate from dark matter annihilation is large enough to shut
off the nuclear burning in the star. Main sequence WIMP burners look much like
protostars moving on the Hayashi track, although they are in principle
completely stable. We make some brief comments about where such stars could be
found, how they might be observed and more detailed simulations which are
currently in progress. Finally we comment on whether or not it is possible to
link the paradoxically young OB stars found at the galactic centre with WIMP
burners.Comment: 4 pages, 3 figs. Matches published versio
Spatial Modulation Microscopy for Real-Time Imaging of Plasmonic Nanoparticles and Cells
Spatial modulation microscopy is a technique originally developed for
quantitative spectroscopy of individual nano-objects. Here, a parallel
implementation of the spatial modulation microscopy technique is demonstrated
based on a line detector capable of demodulation at kHz frequencies. The
capabilities of the imaging system are shown using an array of plasmonic
nanoantennas and dendritic cells incubated with gold nanoparticles.Comment: 3 pages, 4 figure
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