922 research outputs found
Moduli stabilization in (string) model building: gauge fluxes and loops
We discuss the moduli stabilization arising in the presence of gauge fluxes,
R-symmetry twists and non-perturbative effects in the context of 6-dimensional
supergravity models. We show how the presence of D-terms, due to the gauge
fluxes, is compatible with gaugino condensation, and that the two effects,
combined with the R-symmetry twist, do stabilize all the Kaehler moduli present
in the model, in the spirit of KKLT. We also calculate the flux-induced
one-loop correction to the scalar potential coming from charged
hypermultiplets, and find that it does not destabilize the minimum.Comment: Submitted for the SUSY07 proceedings, 4 pages, LaTe
Gauge unification, non-local breaking, open strings
The issue of non-local GUT symmetry breaking is addressed in the context of
open string model building. We study ZNxZM' orbifolds with all the GUT-breaking
orbifold elements acting freely, as rotations accompanied by translations in
the internal space.We consider open strings quantized on these backgrounds,
distinguishing whether the translational action is parallel or perpendicular to
the D-branes. GUT breaking is impossible in the purely perpendicular case,
non-local GUT breaking is instead allowed in the purely parallel case. In the
latter, the scale of breaking is set by the compactification moduli, and there
are no fixed points with reduced gauge symmetry, where dangerous explicit
GUT-breaking terms could be located. We investigate the mixed
parallel+perpendicular case in a Z2xZ2' example, having also a simplified field
theory realization.It is a new S1/Z2xZ2' orbifold-GUT model, with bulk gauge
symmetry SU(5)xSU(5) broken locally to the Standard Model gauge group. In spite
of the locality of the GUT symmetry breaking, there is no localized
contribution to the running of the coupling constants, and the unification
scale is completely set by the length of S1.Comment: 16 pages, 6 figure
Axions in string theory — slaying the Hydra of dark radiation
It is widely believed that string theory easily allows for a QCD axion in the cosmologically favored mass range. The required small decay constant, f(a) << M-P, can be implemented by using a large compactification volume. This points to the Large Volume Scenario which in turn makes certain cosmological predictions: first, the closed string axion behaves similarly to a field-theoretic axion in the pre-inflationary scenario, i.e. the initial value can be tuned but one is constrained by isocurvature fluctuations. In addition, the volume represents a long-lived modulus that may lead to an early matter-dominated phase. Finally, the decay of the volume modulus to its own axion tends to overproduce dark radiation. In this paper we aim to carefully analyze the cosmology by studying models that not only allow for a QCD axion but also include inflation. Quite generally, limits on isocurvature fluctuations restrict us to relatively low-scale inflation, which in the present stringy context points to Kahler moduli inflation. As a novel feature we find that the lightest (volume) modulus couples strongly to the Higgs. It hence quickly decays to the SM, thus resolving the original dark radiation problem. This decay is much faster than that of the inflaton, implying that reheating is determined by the inflaton decay. The inflaton could potentially reintroduce a dark radiation problem since it decays to lighter moduli and their axions with equal rates. However, due its mixing with the QCD-saxion, the inflaton has also a direct decay rate to the SM, enhanced by the number of SM gauge bosons. This results in an amount of dark radiation that is consistent with present limits but potentially detectable in future measurements
Chiral Compactification on a Square
We study quantum field theory in six dimensions with two of them compactified
on a square. A simple boundary condition is the identification of two pairs of
adjacent sides of the square such that the values of a field at two identified
points differ by an arbitrary phase. This allows a chiral fermion content for
the four-dimensional theory obtained after integrating over the square. We find
that nontrivial solutions for the field equations exist only when the phase is
a multiple of \pi/2, so that this compactification turns out to be equivalent
to a T^2/Z_4 orbifold associated with toroidal boundary conditions that are
either periodic or anti-periodic. The equality of the Lagrangian densities at
the identified points in conjunction with six-dimensional Lorentz invariance
leads to an exact Z_8\times Z_2 symmetry, where the Z_2 parity ensures the
stability of the lightest Kaluza-Klein particle.Comment: 28 pages, latex. References added. Clarifying remarks included in
section 2. Minor corrections made in section
The MSSM from Scherk-Schwarz Supersymmetry Breaking
We present a five-dimensional model compactified on an interval where
supersymmetry is broken by the Scherk-Schwarz mechanism. The gauge sector
propagates in the bulk, two Higgs hypermultiplets are quasilocalized, and quark
and lepton multiplets localized, in one of the boundaries. The effective
four-dimensional theory is the MSSM with very heavy gauginos, heavy squarks and
light sleptons and Higgsinos. The soft tree-level squared masses of the Higgs
sector can be negative and they can (partially) cancel the positive one-loop
contributions from the gauge sector. Electroweak symmetry breaking can then
comfortably be triggered by two-loop radiative corrections from the top-stop
sector. The fine tuning required to obtain the electroweak scale is found to be
much smaller than in the MSSM, with essentially no fine-tuning for few TeV
gaugino masses. All bounds from direct Higgs searches at LEP and from
electroweak precision observables can be satisfied. The lightest supersymmetric
particle is a (Higgsino-like) neutralino that can accomodate the abundance of
Dark Matter consistently with recent WMAP observations.Comment: 23 pages, 3 figure
The order of the phase transition in 3d U(1)+Higgs theory
We study the order of the phase transition in the 3d U(1)+Higgs theory, which
is the Ginzburg-Landau theory of superconductivity. We confirm that for small
scalar self-coupling the transition is of first order. For large scalar
self-coupling the transition ceases to be of first order, and a non-vanishing
scalar mass suggests that the transition may even be of higher than second
order.Comment: Poster at LATTICE96(electroweak). 4 pages, 5 figure
Gauge Unification in Highly Anisotropic String Compactifications
It is well-known that heterotic string compactifications have, in spite of
their conceptual simplicity and aesthetic appeal, a serious problem with
precision gauge coupling unification in the perturbative regime of string
theory. Using both a duality-based and a field-theoretic definition of the
boundary of the perturbative regime, we reevaluate the situation in a
quantitative manner. We conclude that the simplest and most promising
situations are those where some of the compactification radii are exceptionally
large, corresponding to highly anisotropic orbifold models. Thus, one is led to
consider constructions which are known to the effective field-theorist as
higher-dimensional or orbifold grand unified theories (orbifold GUTs). In
particular, if the discrete symmetry used to break the GUT group acts freely, a
non-local breaking in the larger compact dimensions can be realized, leading to
a precise gauge coupling unification as expected on the basis of the MSSM
particle spectrum. Furthermore, a somewhat more model dependent but
nevertheless very promising scenario arises if the GUT breaking is restricted
to certain singular points within the manifold spanned by the larger
compactification radii.Comment: 34 pages, 4 figures, more references adde
Realization of within the differntial algebra on
We realize the Hopf algebra as an algebra of differential
operators on the quantum Euclidean space . The generators are
suitable q-deformed analogs of the angular momentum components on ordinary
. The algebra of functions on
splits into a direct sum of irreducible vector representations of
; the latter are explicitly constructed as highest weight
representations.Comment: 26 pages, 1 figur
Energy Transfer between Throats from a 10d Perspective
Strongly warped regions, also known as throats, are a common feature of the
type IIB string theory landscape. If one of the throats is heated during
cosmological evolution, the energy is subsequently transferred to other throats
or to massless fields in the unwarped bulk of the Calabi-Yau orientifold. This
energy transfer proceeds either by Hawking radiation from the black hole
horizon in the heated throat or, at later times, by the decay of
throat-localized Kaluza-Klein states. In both cases, we calculate in a 10d
setup the energy transfer rate (respectively decay rate) as a function of the
AdS scales of the throats and of their relative distance. Compared to existing
results based on 5d models, we find a significant suppression of the energy
transfer rates if the size of the embedding Calabi-Yau orientifold is much
larger than the AdS radii of the throats. This effect can be partially
compensated by a small distance between the throats. These results are
relevant, e.g., for the analysis of reheating after brane inflation. Our
calculation employs the dual gauge theory picture in which each throat is
described by a strongly coupled 4d gauge theory, the degrees of freedom of
which are localized at a certain position in the compact space.Comment: 25 pages; a comment adde
Family Unification on an Orbifold
We construct a family-unified model on a Z_2xZ_2 orbifold in five dimensions.
The model is based on a supersymmetric SU(7) gauge theory. The gauge group is
broken by orbifold boundary conditions to a product of grand unified SU(5) and
SU(2)xU(1) flavor symmetry. The structure of Yukawa matrices is generated by an
interplay between spontaneous breaking of flavor symmetry and geometric factors
arising due to field localization in the extra dimension.Comment: 13 page
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