106 research outputs found
The Neutrino Suppression Scale from Large Volumes
We present an argument in which the scale ~ 0.1 eV associated with neutrino
masses naturally appears in a a class of (very) large volume compactifications,
being tied to a supersymmetry scale of 10^3 GeV and a string scale of 10^11
GeV. The masses are of Majorana type and there is no right-handed neutrino
within the low-energy field theory. The suppression scale 10^14 GeV is
independent of the masses of the heavy states that are integrated out. These
kind of constructions appear naturally in Type IIB flux compactifications.
However, the arguments that lead to this result rely only on a few geometrical
features of the compactification manifold, and hence can be used independently
of string theory.Comment: 4 pages, RevTeX; v2. matches journal versio
Gauge Threshold Corrections for Local String Models
We study gauge threshold corrections for local brane models embedded in a
large compact space. A large bulk volume gives important contributions to the
Konishi and super-Weyl anomalies and the effective field theory analysis
implies the unification scale should be enhanced in a model-independent way
from M_s to R M_s. For local D3/D3 models this result is supported by the
explicit string computations. In this case the scale R M_s comes from the
necessity of global cancellation of RR tadpoles sourced by the local model. We
also study D3/D7 models and discuss discrepancies with the effective field
theory analysis. We comment on phenomenological implications for gauge coupling
unification and for the GUT scale.Comment: 30 pages; v2: references added, minor typos correcte
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
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
Quantum Gravity Constraints on Inflation
We study quantum gravity constraints on inflationary model building. Our
approach is based on requiring the entropy associated to a given inflationary
model to be less than that of the de Sitter entropy. We give two prescriptions
for determining the inflationary entropy, based on either `bits per unit area'
or entanglement entropy. The existence of transPlanckian flat directions,
necessary for large tensor modes in the CMB, correlates with an inflationary
entropy greater than that allowed by de Sitter space. Independently these
techniques also constrain or exclude de Sitter models with large-rank gauge
groups and high UV cutoffs, such as racetrack inflation or the KKLT
construction.Comment: 22 pages; v2 references adde
Moduli Redefinitions and Moduli Stabilisation
Field redefinitions occur in string compactifications at the one loop level.
We review arguments for why such redefinitions occur and study their effect on
moduli stabilisation and supersymmetry breaking in the LARGE volume scenario.
For small moduli, although the effect of such redefinitions can be larger than
that of the corrections in both the K\"ahler and scalar potentials,
they do not alter the structure of the scalar potential. For the less well
motivated case of large moduli, the redefinitions can dominate all other terms
in the scalar potential. We also study the effect of redefinitions on the
structure of supersymmetry breaking and soft terms.Comment: 21 pages, 3 figures; v2. references adde
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
Astrophysical and Cosmological Implications of Large Volume String Compactifications
We study the spectrum, couplings and cosmological and astrophysical
implications of the moduli fields for the class of Calabi-Yau IIB string
compactifications for which moduli stabilisation leads to an exponentially
large volume V ~ 10^{15} l_s^6 and an intermediate string scale m_s ~
10^{11}GeV, with TeV-scale observable supersymmetry breaking. All K\"ahler
moduli except for the overall volume are heavier than the susy breaking scale,
with m ~ ln(M_P/m_{3/2}) m_{3/2} ~ (\ln(M_P/m_{3/2}))^2 m_{susy} ~ 500 TeV and,
contrary to standard expectations, have matter couplings suppressed only by the
string scale rather than the Planck scale. These decay to matter early in the
history of the universe, with a reheat temperature T ~ 10^7 GeV, and are free
from the cosmological moduli problem (CMP). The heavy moduli have a branching
ratio to gravitino pairs of 10^{-30} and do not suffer from the gravitino
overproduction problem. The overall volume modulus is a distinctive feature of
these models and is an M_{planck}-coupled scalar of mass m ~ 1 MeV and subject
to the CMP. A period of thermal inflation can help relax this problem. This
field has a lifetime ~ 10^{24}s and can contribute to dark matter. It may be
detected through its decays to 2\gamma or e^+e^-. If accessible the e^+e^-
decay mode dominates, with Br(\chi \to 2 \gamma) suppressed by a factor
(ln(M_P/m_{3/2}))^2. We consider the potential for detection of this field
through different astrophysical sources and find that the observed gamma-ray
background constrains \Omega_{\chi} <~ 10^{-4}. The decays of this field may
generate the 511 keV emission line from the galactic centre observed by
INTEGRAL/SPI.Comment: 31 pages, 2 figures; v2. refs adde
Random walk representations of the Heisenberg model
We develop random walk representations for the spin- S Heisenberg ferromagnet with nearest neighbor interactions. We show that the spin- S Heisenberg model is a diffusion with local times controlled by the spin- S Ising model. As a consequence, expectations for the Heisenberg model conditioned on zero diffusion are shown to be Ising expectations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45160/1/10955_2005_Article_BF01057876.pd
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