214 research outputs found
Fermion Energies in the Background of a Cosmic String
We provide a thorough exposition, including technical and numerical details,
of previously published results on the quantum stabilization of cosmic strings.
Stabilization occurs through the coupling to a heavy fermion doublet in a
reduced version of the standard model. We combine the vacuum polarization
energy of fermion zero-point fluctuations and the binding energy of occupied
energy levels, which are of the same order in a semi-classical expansion.
Populating these bound states assigns a charge to the string. We show that
strings carrying fermion charge become stable if the electro-weak bosons are
coupled to a fermion that is less than twice as heavy as the top quark. The
vacuum remains stable in our model, because neutral strings are not
energetically favored. These findings suggests that extraordinarily large
fermion masses or unrealistic couplings are not required to bind a cosmic
string in the standard model.Comment: 38 pages, 6 figures, version accepted for publication in Phys Rev
Abelian Higgs Hair for Electrically Charged Dilaton Black Holes
It is argued that an electronically charged dilaton black hole can support a
long range field of a Nielsen-Olesen string. Combining both numerical and
perturbative techniques we examine the properties of an Abelian-Higgs vortex in
the presence of the black hole under consideration. Allowing the black hole to
approach extremality we found that all fields of the vortex are expelled from
the extreme black hole. In the thin string limit we obtained the metric of a
conical electrically charged dilaton black hole. The effect of the vortex can
be measured from infinity justifying its characterization as black hole hair.Comment: 13 pages, 14 figures, Revtex, to appear in Phys.Rev.D1
On the Effective Description of Large Volume Compactifications
We study the reliability of the Two-Step moduli stabilization in the type-IIB
Large Volume Scenarios with matter and gauge interactions. The general analysis
is based on a family of N=1 Supergravity models with a factorizable Kaehler
invariant function, where the decoupling between two sets of fields without a
mass hierarchy is easily understood. For the Large Volume Scenario particular
analyses are performed for explicit models, one of such developed for the first
time here, finding that the simplified version, where the Dilaton and Complex
structure moduli are regarded as frozen by a previous stabilization, is a
reliable supersymmetric description whenever the neglected fields stand at
their leading F-flatness conditions and be neutral. The terms missed by the
simplified approach are either suppressed by powers of the Calabi-Yau volume,
or are higher order operators in the matter fields, and then irrelevant for the
moduli stabilization rocedure. Although the power of the volume suppressing
such corrections depends on the particular model, up to the mass level it is
independent of the modular weight for the matter fields. This at least for the
models studied here but we give arguments to expect the same in general. These
claims are checked through numerical examples. We discuss how the factorizable
models present a context where despite the lack of a hierarchy with the
supersymmetry breaking scale, the effective theory still has a supersymmetric
description. This can be understood from the fact that it is possible to find
vanishing solution for the auxiliary components of the fields being integrated
out, independently of the remaining dynamics. Our results settle down the
question on the reliability of the way the Dilaton and Complex structure are
treated in type-IIB compactifications with large compact manifold volumina.Comment: 23 pages + 2 appendices (38 pages total). v2: minor improvements,
typos fixed. Version published in JHE
Quantum Mechanics of a Point Particle in 2+1 Dimensional Gravity
We study the phase space structure and the quantization of a pointlike
particle in 2+1 dimensional gravity. By adding boundary terms to the first
order Einstein Hilbert action, and removing all redundant gauge degrees of
freedom, we arrive at a reduced action for a gravitating particle in 2+1
dimensions, which is invariant under Lorentz transformations and a group of
generalized translations. The momentum space of the particle turns out to be
the group manifold SL(2). Its position coordinates have non-vanishing Poisson
brackets, resulting in a non-commutative quantum spacetime. We use the
representation theory of SL(2) to investigate its structure. We find a
discretization of time, and some semi-discrete structure of space. An
uncertainty relation forbids a fully localized particle. The quantum dynamics
is described by a discretized Klein Gordon equation.Comment: 58 pages, 3 eps figures, presentation of the classical theory
improve
Two particle Quantummechanics in 2+1 Gravity using Non Commuting Coordinates
We find that the momentum conjugate to the relative distance between two
gravitating particles in their center of mass frame is a hyperbolic angle. This
fact strongly suggests that momentum space should be taken to be a hyperboloid.
We investigate the effect of quantization on this curved momentum space. The
coordinates are represented by non commuting, Hermitian operators on this
hyperboloid. We also find that there is a smallest distance between the two
particles of one half times the Planck length.Comment: 18 pages Latex, 2 eps figure
Supersymmetric Vacua in Random Supergravity
We determine the spectrum of scalar masses in a supersymmetric vacuum of a
general N=1 supergravity theory, with the Kahler potential and superpotential
taken to be random functions of N complex scalar fields. We derive a random
matrix model for the Hessian matrix and compute the eigenvalue spectrum.
Tachyons consistent with the Breitenlohner-Freedman bound are generically
present, and although these tachyons cannot destabilize the supersymmetric
vacuum, they do influence the likelihood of the existence of an `uplift' to a
metastable vacuum with positive cosmological constant. We show that the
probability that a supersymmetric AdS vacuum has no tachyons is formally
equivalent to the probability of a large fluctuation of the smallest eigenvalue
of a certain real Wishart matrix. For normally-distributed matrix entries and
any N, this probability is given exactly by P = exp(-2N^2|W|^2/m_{susy}^2),
with W denoting the superpotential and m_{susy} the supersymmetric mass scale;
for more general distributions of the entries, our result is accurate when N >>
1. We conclude that for |W| \gtrsim m_{susy}/N, tachyonic instabilities are
ubiquitous in configurations obtained by uplifting supersymmetric vacua.Comment: 26 pages, 6 figure
Quantum Energies of Strings in a 2+1 Dimensional Gauge Theory
We study classically unstable string type configurations and compute the
renormalized vacuum polarization energies that arise from fermion fluctuations
in a 2+1 dimensional analog of the standard model. We then search for a minimum
of the total energy (classical plus vacuum polarization energies) by varying
the profile functions that characterize the string. We find that typical string
configurations bind numerous fermions and that populating these levels is
beneficial to further decrease the total energy. Ultimately our goal is to
explore the stabilization of string type configurations in the standard model
through quantum effects.
We compute the vacuum polarization energy within the phase shift formalism
which identifies terms in the Born series for scattering data and Feynman
diagrams. This approach allows us to implement standard renormalization
conditions of perturbation theory and thus yields the unambiguous result for
this non--perturbative contribution to the total energy.Comment: 26 pages, 20 eps-files combined to 8 figures, minor typos corrected.
Version to be published in Nucl. Phys.
Solitons in Supersymmety Breaking Meta-Stable Vacua
In recently found supersymmetry-breaking meta-stable vacua of the
supersymmetric QCD, we examine possible exsitence of solitons. Homotopy groups
of the moduli space of the meta-stable vacua show that there is no nontrivial
soliton for SU(N_c) gauge group. When U(1)_B symmetry present in the theory is
gauged, we find non-BPS solitonic (vortex) strings whose existence and
properties are predicted from brane configurations. We obtain explicit
classical solutions which reproduce the predicitions. For SO(N_c) gauge group,
we find there are solitonic strings for N = N_f-N_c+4 = 2, and Z_2 strings for
the other N. The strings are meta-stable as they live in the meta-stable vacua.Comment: 30 pages, 14 figures, Comments on stability of non-BPS vortices are
added, Comments on sigma model solitons are added, An appendix is adde
Exact Four-Dimensional Dyonic Black Holes and Bertotti-Robinson Spacetimes in String Theory
Conformal field theories corresponding to two-dimensional electrically
charged black holes and to two-dimensional anti-de Sitter space with a
covariantly constant electric field are simply constructed as WZW
coset models. The two-dimensional electrically charged black holes are related
by Kaluza-Klein reduction to the 2+1-dimensional rotating black hole of
Banados, Teitelboim and Zanelli, and our construction is correspondingly
related to its realization as a WZW model. Four-dimensional spacetime solutions
are obtained by tensoring these two-dimensional theories with
coset models. These describe a family of dyonic black holes and the
Bertotti--Robinson universe.Comment: 10 pages, harvmac, (Reference to Kaloper added.
NS5-Branes, T-Duality and Worldsheet Instantons
The equivalence of NS5-branes and ALF spaces under T-duality is well known.
However, a naive application of T-duality transforms the ALF space into a
smeared NS5-brane, de-localized on the dual, transverse, circle. In this paper
we re-examine this duality, starting from a two-dimensional N=(4,4) gauged
linear sigma model describing Taub-NUT space. After dualizing the circle fiber,
we find that the smeared NS5-brane target space metric receives corrections
from multi-worldsheet instantons. These instantons are identified as
Nielsen-Olesen vortices. We show that their effect is to break the isometry of
the target space, localizing the NS5-brane at a point. The contribution from
the k-instanton sector is shown to be proportional to the weighted integral of
the Euler form over the k-vortex moduli space. The duality also predicts the,
previously unknown, asymptotic exponential decay coefficient of the BPS vortex
solution.Comment: 26 pages. v2: Fourier modes of multi-vortex fermion zero mode
corrected. Reference added. v3: typo correcte
- …