206 research outputs found
Scanning the Landscape of Flux Compactifications: Vacuum Structure and Soft Supersymmetry Breaking
We scan the landscape of flux compactifications for the Calabi-Yau manifold
with two K\" ahler moduli by varying the value of
the flux superpotential over a large range of values. We do not include
uplift terms. We find a rich phase structure of AdS and dS vacua. Starting with
we reproduce the exponentially large volume scenario, but as
is reduced new classes of minima appear. One of them corresponds to the
supersymmetric KKLT vacuum while the other is a new, deeper non-supersymmetric
minimum. We study how the bare cosmological constant and the soft supersymmetry
breaking parameters for matter on D7 branes depend on , for these classes
of minima. We discuss potential applications of our results.Comment: draft format remove
Systematics of String Loop Corrections in Type IIB Calabi-Yau Flux Compactifications
We study the behaviour of the string loop corrections to the N=1 4D
supergravity Kaehler potential that occur in flux compactifications of IIB
string theory on general Calabi-Yau three-folds. We give a low energy
interpretation for the conjecture of Berg, Haack and Pajer for the form of the
loop corrections to the Kaehler potential. We check the consistency of this
interpretation in several examples. We show that for arbitrary Calabi-Yaus, the
leading contribution of these corrections to the scalar potential is always
vanishing, giving an "extended no-scale structure". This result holds as long
as the corrections are homogeneous functions of degree -2 in the 2-cycle
volumes. We use the Coleman-Weinberg potential to motivate this cancellation
from the viewpoint of low-energy field theory. Finally we give a simple formula
for the 1-loop correction to the scalar potential in terms of the tree-level
Kaehler metric and the correction to the Kaehler potential. We illustrate our
ideas with several examples. A companion paper will use these results in the
study of Kaehler moduli stabilisation.Comment: 34 pages and 3 figures; typos corrected and references adde
Towards Realistic String Vacua From Branes At Singularities
We report on progress towards constructing string models incorporating both
realistic D-brane matter content and moduli stabilisation with dynamical
low-scale supersymmetry breaking. The general framework is that of local
D-brane models embedded into the LARGE volume approach to moduli stabilisation.
We review quiver theories on del Pezzo () singularities including
both D3 and D7 branes. We provide supersymmetric examples with three
quark/lepton families and the gauge symmetries of the Standard, Left-Right
Symmetric, Pati-Salam and Trinification models, without unwanted chiral
exotics. We describe how the singularity structure leads to family symmetries
governing the Yukawa couplings which may give mass hierarchies among the
different generations. We outline how these models can be embedded into compact
Calabi-Yau compactifications with LARGE volume moduli stabilisation, and state
the minimal conditions for this to be possible. We study the general structure
of soft supersymmetry breaking. At the singularity all leading order
contributions to the soft terms (both gravity- and anomaly-mediation) vanish.
We enumerate subleading contributions and estimate their magnitude. We also
describe model-independent physical implications of this scenario. These
include the masses of anomalous and non-anomalous U(1)'s and the generic
existence of a new hyperweak force under which leptons and/or quarks could be
charged. We propose that such a gauge boson could be responsible for the ghost
muon anomaly recently found at the Tevatron's CDF detector.Comment: 40 pages, 10 figure
LARGE Volume String Compactifications at Finite Temperature
We present a detailed study of the finite-temperature behaviour of the LARGE
Volume type IIB flux compactifications. We show that certain moduli can
thermalise at high temperatures. Despite that, their contribution to the
finite-temperature effective potential is always negligible and the latter has
a runaway behaviour. We compute the maximal temperature , above which
the internal space decompactifies, as well as the temperature , that is
reached after the decay of the heaviest moduli. The natural constraint
implies a lower bound on the allowed values of the internal
volume . We find that this restriction rules out a significant
range of values corresponding to smaller volumes of the order , which lead to standard GUT theories. Instead, the bound favours
values of the order , which lead to TeV scale
SUSY desirable for solving the hierarchy problem. Moreover, our result favours
low-energy inflationary scenarios with density perturbations generated by a
field, which is not the inflaton. In such a scenario, one could achieve both
inflation and TeV-scale SUSY, although gravity waves would not be observable.
Finally, we pose a two-fold challenge for the solution of the cosmological
moduli problem. First, we show that the heavy moduli decay before they can
begin to dominate the energy density of the Universe. Hence they are not able
to dilute any unwanted relics. And second, we argue that, in order to obtain
thermal inflation in the closed string moduli sector, one needs to go beyond
the present EFT description.Comment: 54 pages + appendix, 5 figures; v2: minor corrections, references and
footnotes added, version published on JCA
Sparticle Spectra and LHC Signatures for Large Volume String Compactifications
We study the supersymmetric particle spectra and LHC collider observables for
the large-volume string models with a fundamental scale of 10^{11} GeV that
arise in moduli-fixed string compactifications with branes and fluxes. The
presence of magnetic fluxes on the brane world volume, required for chirality,
perturb the soft terms away from those previously computed in the dilute-flux
limit. We use the difference in high-scale gauge couplings to estimate the
magnitude of this perturbation and study the potential effects of the magnetic
fluxes by generating many random spectra with the soft terms perturbed around
the dilute flux limit. Even with a 40% variation in the high-scale soft terms
the low-energy spectra take a clear and predictive form. The resulting spectra
are broadly similar to those arising on the SPS1a slope, but more degenerate.
In their minimal version the models predict the ratios of gaugino masses to be
M_1 : M_2 : M_3=(1.5 - 2) : 2 : 6, different to both mSUGRA and mirage
mediation. Among the scalars, the squarks tend to be lighter and the sleptons
heavier than for comparable mSUGRA models. We generate 10 fb^{-1} of sample LHC
data for the random spectra in order to study the range of collider
phenomenology that can occur. We perform a detailed mass reconstruction on one
example large-volume string model spectrum. 100 fb^{-1} of integrated
luminosity is sufficient to discriminate the model from mSUGRA and aspects of
the sparticle spectrum can be accurately reconstructed.Comment: 42 pages, 21 figures. Added references and discussion for section 3.
Slight changes in the tex
SUSY Breaking in Local String/F-Theory Models
We investigate bulk moduli stabilisation and supersymmetry breaking in local
string/F-theory models where the Standard Model is supported on a del Pezzo
surface or singularity. Computing the gravity mediated soft terms on the
Standard Model brane induced by bulk supersymmetry breaking in the LARGE volume
scenario, we explicitly find suppressions by M_s/M_P ~ V^{-1/2} compared to
M_{3/2}. This gives rise to several phenomenological scenarios, depending on
the strength of perturbative corrections to the effective action and the source
of de Sitter lifting, in which the soft terms are suppressed by at least
M_P/V^{3/2} and may be as small as M_P/V^2. Since the gravitino mass is of
order M_{3/2} ~ M_P/V, for TeV soft terms all these scenarios give a very heavy
gravitino (M_{3/2} >= 10^8 GeV) and generically the lightest moduli field is
also heavy enough (m >= 10 TeV) to avoid the cosmological moduli problem. For
TeV soft terms, these scenarios predict a minimal value of the volume to be V ~
10^{6-7} in string units, which would give a unification scale of order M_{GUT}
~ M_s V^{1/6} ~ 10^{16} GeV. The strong suppression of gravity mediated soft
terms could also possibly allow a scenario of dominant gauge mediation in the
visible sector but with a very heavy gravitino M_{3/2} > 1 TeV
Natural Quintessence in String Theory
We introduce a natural model of quintessence in string theory where the light
rolling scalar is radiatively stable and couples to Standard Model matter with
weaker-than- Planckian strength. The model is embedded in an anisotropic type
IIB compactification with two exponentially large extra dimensions and
TeV-scale gravity. The bulk turns out to be nearly supersymmetric since the
scale of the gravitino mass is of the order of the observed value of the
cosmological constant. The quintessence field is a modulus parameterising the
size of an internal four-cycle which naturally develops a potential of the
order (gravitino mass)^4, leading to a small dark energy scale without tunings.
The mass of the quintessence field is also radiatively stable since it is
protected by supersymmetry in the bulk. Moreover, this light scalar couples to
ordinary matter via its mixing with the volume mode. Due to the fact that the
quintessence field is a flat direction at leading order, this mixing is very
small, resulting in a suppressed coupling to Standard Model particles which
avoids stringent fifth-force constraints. On the other hand, if dark matter is
realised in terms of Kaluza-Klein states, unsuppressed couplings between dark
energy and dark matter can emerge, leading to a scenario of coupled
quintessence within string theory. We study the dynamics of quintessence in our
set-up, showing that its main features make it compatible with observations.Comment: 26 page
Metastable SUSY Breaking, de Sitter Moduli Stabilisation and K\"ahler Moduli Inflation
We study the influence of anomalous U(1) symmetries and their associated
D-terms on the vacuum structure of global field theories once they are coupled
to N=1 supergravity and in the context of string compactifications with moduli
stabilisation. In particular, we focus on a IIB string motivated construction
of the ISS scenario and examine the influence of one additional U(1) symmetry
on the vacuum structure. We point out that in the simplest one-Kahler modulus
compactification, the original ISS vacuum gets generically destabilised by a
runaway behaviour of the potential in the modulus direction. In more general
compactifications with several Kahler moduli, we find a novel realisation of
the LARGE volume scenario with D-term uplifting to de Sitter space and both
D-term and F-term supersymmetry breaking. The structure of soft supersymmetry
breaking terms is determined in the preferred scenario where the standard model
cycle is not stabilised non-perturbatively and found to be flavour universal.
Our scenario also provides a purely supersymmetric realisation of Kahler moduli
(blow-up and fibre) inflation, with similar observational properties as the
original proposals but without the need to include an extra (non-SUSY)
uplifting term.Comment: 38 pages, 8 figures. v2: references added, minor correction
Gauge Threshold Corrections for Local Orientifolds
We study gauge threshold corrections for systems of fractional branes at
local orientifold singularities and compare with the general Kaplunovsky-Louis
expression for locally supersymmetric N=1 gauge theories. We focus on branes at
orientifolds of the C^3/Z_4, C^3/Z_6 and C^3/Z_6' singularities. We provide a
CFT construction of these theories and compute the threshold corrections. Gauge
coupling running undergoes two phases: one phase running from the bulk winding
scale to the string scale, and a second phase running from the string scale to
the infrared. The first phase is associated to the contribution of N=2 sectors
to the IR beta functions and the second phase to the contribution of both N=1
and N=2 sectors. In contrast, naive application of the Kaplunovsky-Louis
formula gives single running from the bulk winding mode scale. The discrepancy
is resolved through 1-loop non-universality of the holomorphic gauge couplings
at the singularity, induced by a 1-loop redefinition of the twisted blow-up
moduli which couple differently to different gauge nodes. We also study the
physics of anomalous and non-anomalous U(1)s and give a CFT description of how
masses for non-anomalous U(1)s depend on the global properties of cycles.Comment: 44 page
F-theory, GUTs, and the Weak Scale
In this paper we study a deformation of gauge mediated supersymmetry breaking
in a class of local F-theory GUT models where the scale of supersymmetry
breaking determines the value of the mu term. Geometrically correlating these
two scales constrains the soft SUSY breaking parameters of the MSSM. In this
scenario, the hidden SUSY breaking sector involves an anomalous U(1)
Peccei-Quinn symmetry which forbids bare mu and B mu terms. This sector
typically breaks supersymmetry at the desired range of energy scales through a
simple stringy hybrid of a Fayet and Polonyi model. A variant of the
Giudice-Masiero mechanism generates the value mu ~ 10^2 - 10^3 GeV when the
hidden sector scale of supersymmetry breaking is F^(1/2) ~ 10^(8.5) GeV.
Further, the B mu problem is solved due to the mild hierarchy between the GUT
scale and Planck scale. These models relate SUSY breaking with the QCD axion,
and solve the strong CP problem through an axion with decay constant f_a ~
M_(GUT) * mu / L, where L ~ 10^5 GeV is the characteristic scale of gaugino
mass unification in gauge mediated models, and the ratio \mu / L ~
M_(GUT)/M_(pl) ~ 10^(-3). We find f_a ~ 10^12 GeV, which is near the high end
of the phenomenologically viable window. Here, the axino is the goldstino mode
which is eaten by the gravitino. The gravitino is the LSP with a mass of about
10^1 - 10^2 MeV, and a bino-like neutralino is (typically) the NLSP with mass
of about 10^2 - 10^3 GeV. Compatibility with electroweak symmetry breaking also
determines the value of tan(beta) ~ 30 +/- 7.Comment: v3: 94 pages, 9 figures, clarification of Fayet-Polonyi model and
instanton corrections to axion potentia
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