1,730 research outputs found
Wilson loops, geometric operators and fermions in 3d group field theory
Group field theories whose Feynman diagrams describe 3d gravity with a
varying configuration of Wilson loop observables and 3d gravity with volume
observables at each vertex are defined. The volume observables are created by
the usual spin network grasping operators which require the introduction of
vector fields on the group. We then use this to define group field theories
that give a previously defined spin foam model for fermion fields coupled to
gravity, and the simpler quenched approximation, by using tensor fields on the
group. The group field theory naturally includes the sum over fermionic loops
at each order of the perturbation theory.Comment: 13 pages, many figures, uses psfra
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
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
Coupling gauge theory to spinfoam 3d quantum gravity
We construct a spinfoam model for Yang-Mills theory coupled to quantum
gravity in three dimensional riemannian spacetime. We define the partition
function of the coupled system as a power series in g_0^2 G that can be
evaluated order by order using grasping rules and the recoupling theory. With
respect to previous attempts in the literature, this model assigns the
dynamical variables of gravity and Yang-Mills theory to the same simplices of
the spinfoam, and it thus provides transition amplitudes for the spin network
states of the canonical theory. For SU(2) Yang-Mills theory we show explicitly
that the partition function has a semiclassical limit given by the Regge
discretization of the classical Yang-Mills action.Comment: 18 page
Dark Matter Capture in the First Stars: a Power Source and Limit on Stellar Mass
The annihilation of weakly interacting massive particles can provide an
important heat source for the first (Pop. III) stars, potentially leading to a
new phase of stellar evolution known as a "Dark Star". When dark matter (DM)
capture via scattering off of baryons is included, the luminosity from DM
annihilation may dominate over the luminosity due to fusion, depending on the
DM density and scattering cross-section. The influx of DM due to capture may
thus prolong the lifetime of the Dark Stars. Comparison of DM luminosity with
the Eddington luminosity for the star may constrain the stellar mass of zero
metallicity stars; in this case DM will uniquely determine the mass of the
first stars. Alternatively, if sufficiently massive Pop. III stars are found,
they might be used to bound dark matter properties.Comment: 19 pages, 4 figures, 3 Tables updated captions and graphs, corrected
grammer, and added citations revised for submission to JCA
Relational trauma and its impact on late-adopted children
This paper describes work with two children, placed for late adoption who have suffered relational trauma. The paper explores the long-term consequences of such trauma, which includes problems with affect regulation, difficulties in generalising from one experience to another and shifts between phantasies of omnipotent control and sudden helplessness. Using drawings from one boy's therapy, it is argued that many children adopted at a later age live in two worlds, both internal and external, and internal objects and memories from the past vie with new experiences and representations for ascendancy within the child's mind. Which is more real: the world of the past or the present? The paper describes how these children experienced sudden and troubling shifts in focus as they were catapulted from feeling states belonging to one world to the other. The paper ends with a consideration of how findings from neuroscience may help us to understand these sudden shifts and overall argues for a pulling together of psychoanalytic thinking and child development research findings to support the child in psychotherapy
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
Standard Cosmological Evolution in a Wide Range of f(R) Models
Using techniques from singular perturbation theory, we explicitly calculate
the cosmological evolution in a class of modified gravity models. By
considering the (m)CDTT model, which aims to explain the current acceleration
of the universe with a modification of gravity, we show that Einstein evolution
can be recovered for most of cosmic history in at least one f(R) model. We show
that a standard epoch of matter domination can be obtained in the mCDTT model,
providing a sufficiently long epoch to satisfy observations. We note that the
additional inverse term will not significantly alter standard evolution until
today and that the solution lies well within present constraints from Big Bang
Nucleosynthesis. For the CDTT model, we analyse the ``recent radiation epoch''
behaviour (a \propto t^{1/2}) found by previous authors. We finally generalise
our findings to the class of inverse power-law models. Even in this class of
models, we expect a standard cosmological evolution, with a sufficient matter
domination era, although the sign of the additional term is crucial.Comment: 15 pages, 6 figures (1 new figure), new version considers both CDTT
and mCDTT models. References added. Accepted by Phys Rev
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Ultrasmall silica nanoparticles directly ligate the T cell receptor complex.
The impact of ultrasmall nanoparticles (<10-nm diameter) on the immune system is poorly understood. Recently, ultrasmall silica nanoparticles (USSN), which have gained increasing attention for therapeutic applications, were shown to stimulate T lymphocytes directly and at relatively low-exposure doses. Delineating underlying mechanisms and associated cell signaling will hasten therapeutic translation and is reported herein. Using competitive binding assays and molecular modeling, we established that the T cell receptor (TCR):CD3 complex is required for USSN-induced T cell activation, and that direct receptor complex-particle interactions are permitted both sterically and electrostatically. Activation is not limited to αÎČ TCR-bearing T cells since those with γΎ TCR showed similar responses, implying that USSN mediate their effect by binding to extracellular domains of the flanking CD3 regions of the TCR complex. We confirmed that USSN initiated the signaling pathway immediately downstream of the TCR with rapid phosphorylation of both ζ-chain-associated protein 70 and linker for activation of T cells protein. However, T cell proliferation or IL-2 secretion were only triggered by USSN when costimulatory anti-CD28 or phorbate esters were present, demonstrating that the specific impact of USSN is in initiation of the primary, nuclear factor of activated T cells-pathway signaling from the TCR complex. Hence, we have established that USSN are partial agonists for the TCR complex because of induction of the primary T cell activation signal. Their ability to bind the TCR complex rapidly, and then to dissolve into benign orthosilicic acid, makes them an appealing option for therapies targeted at transient TCR:CD3 receptor binding.These studies were supported by grants from the UK Medical Research Council (Grant number MR/R005699/1) and the Natural Sciences and Engineering Research Council of Canada, as well as through sponsorship from HS Pharmaceuticals LLC
Encoding simplicial quantum geometry in group field theories
We show that a new symmetry requirement on the GFT field, in the context of
an extended GFT formalism, involving both Lie algebra and group elements,
leads, in 3d, to Feynman amplitudes with a simplicial path integral form based
on the Regge action, to a proper relation between the discrete connection and
the triad vectors appearing in it, and to a much more satisfactory and
transparent encoding of simplicial geometry already at the level of the GFT
action.Comment: 15 pages, 2 figures, RevTeX, references adde
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