1,438 research outputs found
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
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
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
Randall-Sundrum black holes and strange stars
It has recently been suggested that the existence of bare strange stars is
incompatible with low scale gravity scenarios. It has been claimed that in such
models, high energy neutrinos incident on the surface of a bare strange star
would lead to catastrophic black hole growth. We point out that for the flat
large extra dimensional case, the parts of parameter space which give rise to
such growth are ruled out by other methods. We then go on to show in detail how
black holes evolve in the the Randall-Sundrum two brane scenario where the
extra dimensions are curved. We find that catastrophic black hole growth does
not occur in this situation either. We also present some general expressions
for the growth of five dimensional black holes in dense media.Comment: 16 pages, more numerics has lead to different path to same
conclusion. Accepted in PR
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
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
Trigger and reconstruction for heavy long-lived charged particles with the ATLAS detector
Long lived charged particles are predicted by many models of physics beyond the standard model (SM). The common signature of such models is a heavy long-lived charged particle with velocity smaller than the speed of light, β<1. This unique signature makes the search for it model independent. This paper presents methods we developed as part of the ATLAS trigger and reconstruction chain for identifying slow particles and measuring their mass. The efficacy of these methods is demonstrated using two models that are different in every aspect except for the existence of long lived charged particles; a GMSB model that includes sleptons with a mass of 100 GeV, and R-Hadrons with a mass of 300 GeV produced in a split SUSY model
Grasping rules and semiclassical limit of the geometry in the Ponzano-Regge model
We show how the expectation values of geometrical quantities in 3d quantum
gravity can be explicitly computed using grasping rules. We compute the volume
of a labelled tetrahedron using the triple grasping. We show that the large
spin expansion of this value is dominated by the classical expression, and we
study the next to leading order quantum corrections.Comment: 18 pages, 1 figur
Minimal Family Unification
Absract It is proposed that there exist, within a new , a gauged
discrete group (the order 12 double dihedral group) acting as a family
symmetry. This nonabelian finite group can explain hierarchical features of
families, using an assignment for quarks and leptons dictated by the
requirements of anomaly cancellation and of no additional quarks.Comment: 10 pages, IFP-701-UNC;VAND-TH-94-
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