576 research outputs found

### Vacuum Instability in Chern-Simons Gravity

We explore perturbations about a Friedmann-Robertson-Walker background in
Chern-Simons gravity. At large momenta one of the two circularly polarized
tensor modes becomes ghostlike. We argue that nevertheless the theory does not
exhibit classical runaway solutions, except possibly in the relativistic
nonlinear regime. However, the ghost modes cause the vacuum state to be quantum
mechanically unstable, with a decay rate that is naively infinite. The decay
rate can be made finite only if one interprets the theory as an effective
quantum field theory valid up to some momentum cutoff, which violates Lorentz
invariance. By demanding that the energy density in photons created by vacuum
decay over the lifetime of the Universe not violate observational bounds, we
derive strong constraints on the two dimensional parameter space of the theory,
consisting of the cutoff and the Chern-Simons mass.Comment: 8 pages, 2 figures; final published versio

### Gravitational Interactions in a General Multibrane Model

The gravitational interactions of the four-dimensional effective theory
describing a general $N$-brane model in five dimensions without radion
stabilization are analyzed. Both uncompactified and orbifolded models are
considered. The parameter space is constrained by requiring that there be no
ghost modes in the theory, and that the Eddington parameterized post-Newtonian
parameter $\gamma$ be consistent with observations. We show that we must reside
on the brane on which the warp factor is maximized. The resultant theory
contains $N-1$ radion modes in a nonlinear sigma model, with the target space
being a subset of hyperbolic space. Imposing observational constraints on the
relative strengths of gravitational interactions of dark and visible matter
shows that at least 99.8% of the dark matter must live on our brane in this
model.Comment: 18 pages, 4 figures. Version 2 (submitted to PRD) adds analysis on
orbifold

### Probing Compressed Top Squarks at the LHC at 14 TeV

A feasibility study is presented for the search of the lightest top squark in
a compressed scenario, where its mass is approximately equal to the sum of the
masses of the top quark and the lightest neutralino. The study is performed in
the final state of two b-jets, one lepton, large missing energy, and two
high-$E_{\rm T}$ jets with large separation in pseudo-rapidity, in opposite
hemispheres, and with large dijet mass. The LHC could discover compressed top
squarks with mass up to approximately 340 GeV (390 GeV) with an integrated
luminosity of 1000 ifb (3000 ifb).Comment: Version updated with major changes: (a) 3-body stop decay (to b+W+n1)
analyzed for first time (b) systematics calculation and discussion
significantly upgraded (c) new kinematic and mass reach plots for the 3-body
decay scenario added (d) discussions clarified throughou

### Search for Supersymmetry Using Weak Boson Fusion Processes in Proton-Proton Collisions at the Large Hadron Collider

In 2012, the Large Hadron Collider at CERN (LHC) collided protons at an unprecedented center-of-mass energy of 8 TeV. With data corresponding to a total integrated luminosity of 19.7 fb^(−1), the Compact Muon Solenoid (CMS) collaboration is studying various Standard Model processes as well as searching for Beyond the Standard Model processes through a plethora of distinct projects. This dissertation
searches for Supersymmetry using weak boson fusion processes. These processes provide a unique opportunity to access the electroweak sector of super-symmetry. Though well motivated, this search finds no excess above the Standard Model. Exclusion limits are obtained in the context of electroweakino masses at the 95% confidence level

### Metric-affine f(R) theories of gravity

General Relativity assumes that spacetime is fully described by the metric
alone. An alternative is the so called Palatini formalism where the metric and
the connections are taken as independent quantities. The metric-affine theory
of gravity has attracted considerable attention recently, since it was shown
that within this framework some cosmological models, based on some generalized
gravitational actions, can account for the current accelerated expansion of the
universe. However we think that metric-affine gravity deserves much more
attention than that related to cosmological applications and so we consider
here metric-affine gravity theories in which the gravitational action is a
general function of the scalar curvature while the matter action is allowed to
depend also on the connection which is not {\em a priori} symmetric. This
general treatment will allow us to address several open issues such as: the
relation between metric-affine $f(R)$ gravity and General Relativity (in vacuum
as well as in the presence of matter), the implications of the dependence (or
independence) of the matter action on the connections, the origin and role of
torsion and the viability of the minimal-coupling principle.Comment: typos corrected, replaced to match published versio

### Modification to the Luminosity Distance Redshift Relation in Modified Gravity Theories

We derive an expression for the luminosity distance as a function of redshift
for a flat Robertson-Walker spacetime perturbed by arbitrary scalar
perturbations possibly produced by a modified gravity theory with two different
scalar perturbation potentials. Measurements of the luminosity distance as
function of redshift provide a constraint on a combination of the scalar
potentials and so they can complement weak lensing and other measurements in
trying to distinguish among the various alternative theories of gravity.Comment: 15 pages, we discuss in more detail how the luminosity distance
expression can be used to differentiate among various theories of gravit

### Gravitational waves from inspiraling compact binaries: Second post-Newtonian waveforms as search templates II

We present further evidence that the second post-Newtonian (pN) approximation
to the gravitational waves emitted by inspiraling compact binaries is
sufficient for the detection of these systems. This is established by comparing
the 2-pN wave forms to signals calculated from black hole perturbation theory.
Results are presented for different detector noise curves. We also discuss the
validity of this type of analysis.Comment: 5 pages, 3 Figures, RevTe

### The Cauchy problem of scalar-tensor theories of gravity

The 3+1 formulation of scalar-tensor theories of gravity (STT) is obtained in
the physical (Jordan) frame departing from the 4+0 covariant field equations.
Contrary to the common belief (folklore), the new system of ADM-like equations
shows that the Cauchy problem of STT is well formulated (in the sense that the
whole system of evolution equations is of first order in the time-derivative).
This is the first step towards a full first order (in time and space)
formulation from which a subsequent hyperbolicity analysis (a well-posedness
determination) can be performed. Several gauge (lapse and shift) conditions are
considered and implemented for STT. In particular, a generalization of the
harmonic gauge for STT allows us to prove the well posedness of the STT using a
second order analysis which is very similar to the one used in general
relativity. Some spacetimes of astrophysical and cosmological interest are
considered as specific applications. Several appendices complement the ideas of
the main part of the paper.Comment: 29 pages Revtex; typos corrected; references added and updated; a
shorter version of this paper was published in Classical and Quantum Gravit

### Measuring gravitational waves from binary black hole coalescences: II. the waves' information and its extraction, with and without templates

We discuss the extraction of information from detected binary black hole
(BBH) coalescence gravitational waves, focusing on the merger phase that occurs
after the gradual inspiral and before the ringdown. Our results are: (1) If
numerical relativity simulations have not produced template merger waveforms
before BBH detections by LIGO/VIRGO, one can band-pass filter the merger waves.
For BBHs smaller than about 40 solar masses detected via their inspiral waves,
the band pass filtering signal to noise ratio indicates that the merger waves
should typically be just barely visible in the noise for initial and advanced
LIGO interferometers. (2) We derive an optimized (maximum likelihood) method
for extracting a best-fit merger waveform from the noisy detector output; one
"perpendicularly projects" this output onto a function space (specified using
wavelets) that incorporates our prior knowledge of the waveforms. An extension
of the method allows one to extract the BBH's two independent waveforms from
outputs of several interferometers. (3) If numerical relativists produce codes
for generating merger templates but running the codes is too expensive to allow
an extensive survey of the merger parameter space, then a coarse survey of this
parameter space, to determine the ranges of the several key parameters and to
explore several qualitative issues which we describe, would be useful for data
analysis purposes. (4) A complete set of templates could be used to test the
nonlinear dynamics of general relativity and to measure some of the binary
parameters. We estimate the number of bits of information obtainable from the
merger waves (about 10 to 60 for LIGO/VIRGO, up to 200 for LISA), estimate the
information loss due to template numerical errors or sparseness in the template
grid, and infer approximate requirements on template accuracy and spacing.Comment: 33 pages, Rextex 3.1 macros, no figures, submitted to Phys Rev

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