1,213 research outputs found
"Big" Divisor D3/D7 Swiss Cheese Phenomenology
We review progress made over the past couple of years in the field of Swiss
Cheese Phenomenology involving a mobile space-time filling D3-brane and
stack(s) of fluxed D7-branes wrapping the "big" (as opposed to the "small")
divisor in (the orientifold of a) Swiss-Cheese Calabi-Yau. The topics reviewed
include reconciliation of large volume cosmology and phenomenology, evaluation
of soft supersymmetry breaking parameters, one-loop RG-flow equations'
solutions for scalar masses, obtaining fermionic (possibly first two
generations' quarks/leptons) mass scales in the O(MeV-GeV)-regime as well as
(first two generations') neutrino masses (and their one-loop RG flow) of around
an eV. The heavy sparticles and the light fermions indicate the possibility of
"split SUSY" large volume scenario.Comment: Invited review for MPLA, 14 pages, LaTe
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
Sequestered Dark Matter
We show that hidden-sector dark matter is a generic feature of the type IIB
string theory landscape and that its lifetime may allow for a discovery through
the observation of very energetic gamma-rays produced in the decay. Throats or,
equivalently, conformally sequestered hidden sectors are common in flux
compactifications and the energy deposited in these sectors can be calculated
if the reheating temperature of the standard model sector is known. Assuming
that throats with various warp factors are available in the compact manifold,
we determine which throats maximize the late-time abundance of sequestered dark
matter. For such throats, this abundance agrees with cosmological data if the
standard model reheating temperature was 10^10 - 10^11 GeV. In two distinct
scenarios, the mass of dark matter particles, i.e. the IR scale of the throat,
is either around 10^5 GeV or around 10^10 GeV. The lifetime and the decay
channels of our dark matter candidates depend crucially on the fact that the
Klebanov-Strassler throat is supersymmetric. Furthermore, the details of
supersymmetry breaking both in the throat and in the visible sector play an
essential role. We identify a number of scenarios where this type of dark
matter can be discovered via gamma-ray observations.Comment: 36 pages, 3 figures; v2: references added, v3: introduction extended
and typos correcte
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
Wavefunctions and the Point of E8 in F-theory
In F-theory GUTs interactions between fields are typically localised at
points of enhanced symmetry in the internal dimensions implying that the
coefficient of the associated operator can be studied using a local
wavefunctions overlap calculation. Some F-theory SU(5) GUT theories may exhibit
a maximum symmetry enhancement at a point to E8, and in this case all the
operators of the theory can be associated to the same point. We take initial
steps towards the study of operators in such theories. We calculate
wavefunctions and their overlaps around a general point of enhancement and
establish constraints on the local form of the fluxes. We then apply the
general results to a simple model at a point of E8 enhancement and calculate
some example operators such as Yukawa couplings and dimension-five couplings
that can lead to proton decay.Comment: 46 page
Superpotential de-sequestering in string models
Non-perturbative superpotential cross-couplings between visible sector matter
and K\"ahler moduli can lead to significant flavour-changing neutral currents
in compactifications of type IIB string theory. Here, we compute corrections to
Yukawa couplings in orbifold models with chiral matter localised on D3-branes
and non-perturbative effects on distant D7-branes. By evaluating a threshold
correction to the D7-brane gauge coupling, we determine conditions under which
the non-perturbative corrections to the Yukawa couplings appear. The flavour
structure of the induced Yukawa coupling generically fails to be aligned with
the tree-flavour structure. We check our results by also evaluating a
correlation function of two D7-brane gauginos and a D3-brane Yukawa coupling.
Finally, by calculating a string amplitude between n hidden scalars and visible
matter we show how non-vanishing vacuum expectation values of distant D7-brane
scalars, if present, may correct visible Yukawa couplings with a flavour
structure that differs from the tree-level flavour structure.Comment: 37 pages + appendices, 8 figure
On two problems in graph Ramsey theory
We study two classical problems in graph Ramsey theory, that of determining
the Ramsey number of bounded-degree graphs and that of estimating the induced
Ramsey number for a graph with a given number of vertices.
The Ramsey number r(H) of a graph H is the least positive integer N such that
every two-coloring of the edges of the complete graph contains a
monochromatic copy of H. A famous result of Chv\'atal, R\"{o}dl, Szemer\'edi
and Trotter states that there exists a constant c(\Delta) such that r(H) \leq
c(\Delta) n for every graph H with n vertices and maximum degree \Delta. The
important open question is to determine the constant c(\Delta). The best
results, both due to Graham, R\"{o}dl and Ruci\'nski, state that there are
constants c and c' such that 2^{c' \Delta} \leq c(\Delta) \leq 2^{c \Delta
\log^2 \Delta}. We improve this upper bound, showing that there is a constant c
for which c(\Delta) \leq 2^{c \Delta \log \Delta}.
The induced Ramsey number r_{ind}(H) of a graph H is the least positive
integer N for which there exists a graph G on N vertices such that every
two-coloring of the edges of G contains an induced monochromatic copy of H.
Erd\H{o}s conjectured the existence of a constant c such that, for any graph H
on n vertices, r_{ind}(H) \leq 2^{c n}. We move a step closer to proving this
conjecture, showing that r_{ind} (H) \leq 2^{c n \log n}. This improves upon an
earlier result of Kohayakawa, Pr\"{o}mel and R\"{o}dl by a factor of \log n in
the exponent.Comment: 18 page
Unification, KK-thresholds and the top Yukawa coupling in F-theory GUTs
In a class of F-theory SU(5) GUTs the low energy chiral mass spectrum is
obtained from rank one fermion mass textures with a hierarchical structure
organised by U(1) symmetries embedded in the exceptional E_8 group. In these
theories chiral fields reside on matter `curves' and the tree level masses are
computed from integrals of overlapping wavefuctions of the particles at the
triple intersection points. This calculation requires knowledge of the exact
form of the wavefuctions. In this work we propose a way to obtain a reliable
estimate of the various quantities which determine the strength of the Yukawa
couplings. We use previous analysis of KK threshold effects to determine the
(ratios of) heavy mass scales of the theory which are involved in the
normalization of the wave functions. We consider similar effects from the
chiral spectrum of these models and discuss possible constraints on the
emerging matter content. In this approach, we find that the Yukawa couplings
can be determined solely from the U(1) charges of the states in the
`intersection' and the torsion which is a topological invariant quantity. We
apply the results to a viable SU(5) model with minimal spectrum which satisfies
all the constraints imposed by our analysis. We use renormalization group
analysis to estimate the top and bottom masses and find that they are in
agreement with the experimental values.Comment: 28 pages, 2 figure
Dark Energy density in models with Split Supersymmetry and degenerate vacua
In N=1 supergravity supersymmetric (SUSY) and non-supersymmetric Minkowski
vacua originating in the hidden sector can be degenerate. In the supersymmetric
phase in flat Minkowski space non-perturbative supersymmetry breakdown may take
place in the observable sector, inducing a non-zero and positive vacuum energy
density. Assuming that such a supersymmetric phase and the phase in which we
live are degenerate, we estimate the value of the cosmological constant. We
argue that the observed value of the dark energy density can be reproduced in
the Split-SUSY scenario of the supersymmetry breaking if the SUSY breaking
scale is of order of 10^{10} GeV.Comment: 20 pages, 1 figure, some minor changes to the text, references adde
On 'Light' Fermions and Proton Stability in 'Big Divisor' D3/D7 Swiss Cheese Phenomenology
Building up on our earlier work [1,2], we show the possibility of generating
"light" fermion mass scales of MeV-GeV range (possibly related to first two
generations of quarks/leptons) as well as eV (possibly related to first two
generations of neutrinos) in type IIB string theory compactified on
Swiss-Cheese orientifolds in the presence of a mobile space-time filling
D3-$brane restricted to (in principle) stacks of fluxed D7-branes wrapping the
"big" divisor \Sigma_B. This part of the paper is an expanded version of the
latter half of section 3 of a published short invited review [3] written up by
one of the authors [AM]. Further, we also show that there are no SUSY GUT-type
dimension-five operators corresponding to proton decay, as well as estimate the
proton lifetime from a SUSY GUT-type four-fermion dimension-six operator to be
10^{61} years. Based on GLSM calculations in [1] for obtaining the geometric
Kaehler potential for the "big divisor", using further the Donaldson's
algorithm, we also briefly discuss in the first of the two appendices,
obtaining a metric for the Swiss-Cheese Calabi-Yau used, that becomes Ricci
flat in the large volume limit.Comment: v2: 1+25 pages, Title modified and text thoroughly expanded including
a brief discussion on obtaining Ricci-flat Swiss Cheese Calabi-Yau metrics
using the Donaldson's algorithm, references added, to appear in EPJ
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